It appears vampires and werewolves have always been at each other's throats and the Underworld franchise offered a glimpse as to why. Since 2003, the horror-action films have delved into the worlds of these creatures and their centuries-old feud. Despite mixed reviews over the years, the franchise has retained a strong cult-like following. At the core of the films is the beautiful but deadly Selene, portrayed by Kate Beckinsale, who is a Death-Dealer trained to hunt and kill Lycans, the ruthless werewolf clan. Aided by her hybrid lover Michael, she uncovers secrets about not only her past, but the true reason behind the war between the species.

In the fourth installment, Underworld: Awakening, humans have learned of the existence of the monsters and attempt to destroy them. A fifth film was released in 2016 and director Len Wiseman has teased he has plans for a sixth, though an official announcement has yet to be made. There have also been rumors of a possible Underworld-inspired television series but there is still speculation regarding casting and potential storylines. As fans anxiously await news regarding this seemingly endless franchise, CBR has compiled a list of 20 behind-the-scenes facts to whet your insatiable appetite.

20 LOVE AT FIRST BITE

While their characters were on opposite sides of a supernatural war, Kate Beckinsale and Michael Sheen were enjoying a loving, long-term relationship. The couple, now separated, share a daughter named Lily Mo who makes an appearance as a young Selene in a flashback scene in the first Underworld film. The pair met during the touring production of The Seagull in 1995; their relationship ending in 2003. To this day, the two remain very close and dedicated to one another.

Beckinsale persuaded director Len Wiseman to cast Sheen in the first Underworld movie as werewolf leader Lucian. After parting ways with Sheen, Beckinsale began a romance with Wiseman, the two marrying in 2004. In November 2015, the couple announced their separation, filing for divorce in 2016.

19 SWITCHING SIDES

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In three of the franchise's films, Michael Sheen portrays the leader of the Lycan clan. As Lucian, his character has the ability to shift between human and werewolf and in Underworld: Rise of the Lycans, develops a forbidden romance with vampire Sonja, which becomes the catalyst for the war between vampires and lycans. Seeing that the growing romance between Selene and Michael mirrors his own past love, he aids them in their revenge against Viktor.

Once deemed as the "most feared and ruthless leader ever to rule the Lycan hordes," Sheen switched sides and appeared in three of the Twilight Saga films as one of the leaders of the vampire coven Volturi. His character, Aro, is a power-hungry, manipulative leader who hides behind a uncomfortably cheery persona. He is determined to keep his race hidden from the humans, whose technological advances over the years threaten their existence.

18 THANKS CANADA

An international co-production between companies based in The United Kingdom, Germany, Hungary, and the United States, the first Underworld film was released in September 2003. Despite negative reviews, the film was considered a "surprise hit," grossing $95 million. The film's sequel was released three years later and has been followed by several other films throughout the years.

Days before its release in the United States, the president of Screen Gems decided to green-light both a sequel and prequel, in the forms of Underworld: Evolution and Underworld: Rise of the Lycans, based on the number of tickets that had been sold in Canada alone. In Evolution, Selene and Michael continue to uncover dark secrets behind the Corvinus bloodline while in Rise of the Lycans, the reason for the feud between the lycan and vampire clans is revealed.

17 BLOODY GOOD BOWIE

Seen as an iconic presence in the world of pop music for over five decades, David Bowie was known for his over-the-top personality. Along with an impressive musical career, Bowie also made various film appearances. Originally, Len Wiseman wanted to include him during the vampire's feast in the first Underworld film but was unable to due to scheduling conflicts.

In 1983, the musician turned actor appeared as a member of the undead in the erotic-horror film The Hunger as a talented cellist from the 18th century who, along with his companion Miriam, prey upon unsuspecting humans by promising them eternal life and love. While he expressed excitement while working on the film, Bowie did reveal he worried the movie was "just perversely bloody." The film received generally mixed reviews and in 2009, Warner Bros. announced a plan for a remake.

16 MATURE AUDIENCES ONLY

According to IMDB.com, the original cut of Underworld: Evolution featured over two hours worth of footage. One scene in particular had to be cut due to Kate Beckinsale's demands and fear the movie would earn an NC-17 rating. The more mature version of the sex scene between Michael and Selene in the storage container was meant to be much longer and contain more nudity from the actress.

The "fade to black" scene was intended to involve Selene reaching climax before straddling Michael to continue having intercourse. Wiseman had to make heavy edits to the scene due to Beckinsale not wanting to be seen fully nude. The director later teased in the commentary for the theatrical cut that the DVD release would involve some of these deleted scenes but, in the end, they continued to be left out.

15 FEELING FANG-TASTIC

In Underworld: Evolution, Tony Curran plays Marcus Corvinus, the creator of the entire vampire species. He shares a fiercely loyal bond with his twin brother William, who is the first werewolf. He is a sadistic immortal who enjoys bringing pain upon his enemies, drinking their blood to gain necessary information to use against them.

According to the actor, he arranged in his contract to be allowed to wear his fangs on set, even when he wasn't filming. Curran also had the pleasure of playing a vampire named Priest alongside Wesley Snipes in Blade 2. In The League of Extraordinary Gentleman, he played Rodney Skinner, also known as the Invisible Man. Skinner is believed to have stolen the formula from the original H.G. Well's character since the film failed to gain the rights to use the author's creation.

14 THE BLACK QUEEN

selene

Along with her roots in Greek mythology, the character of Selene can trace her origin to the X-Men universe. According to Kevin Grevioux, he modeled her after the psychic vampire The Black Queen Selene. Considered to be the oldest living mutant, Selene has the ability to drain people of their life force to extend her own. Among her various powers, she has many physical enhancements including superhuman strength and speed.

Her character was first introduced in New Mutants #9 in 1983 by Chris Claremont. Upon arriving in New York City, she makes contact with the Hellfire Club and announces herself as their new leader. Referred to as The Black Queen, Selene's time as leader proved quite turbulent due to her clear hatred of Sebastian Shaw and desire to be the group's sole reigning authority.

13 AN UNNECESSARY CAT-CALL

Many actresses were considered for the role of Selene. Among them, Mila Jovovich, who would go on to star in the Resident Evil franchise and Rhona Mitra who played Sonja in Underworld: Rise of the Lycans. A somewhat unexpected name in the running for the role was Halle Berry.

Perhaps seeing her in a leather outfit as Storm in the first X-Men movies led to her being a top contender or the fact that she successfully faced down supernatural entities in Gothika. In the end, Halle Berry was passed over and, in 2004, the actress stared in Catwoman, wearing a skin-tight catsuit of her own. The film, loosely based on the iconic DC character, went on to be considered one of the worst movies of all time. Berry's lackluster performance won her a Golden Raspberry in 2005.

12 CROSSOVER WITH RESIDENT EVIL

Fans of supernatural franchises have long been debating which of the two is the more successful: Underworld or Resident Evil. While Underworld deals with the feud between the vampire and werewolf species, Resident Evil follows a group battling an evil corporation whose inventions have triggered a zombie apocalypse. The franchises share strong female leads, bloody battles with monsters, and involve plenty of CG effects.

While promoting Underworld: Blood Wars, producer David Kern revealed that he had thought about bringing the two universes together. Ultimately, he hesitated to do so because he considered audiences were made up of fans of both franchises. When asked during Comic Con 2012 about a potential crossover, Paul W.S Anderson also voiced reluctance, stating the franchises are owned by two separate companies.

11 CROSSOVER WITH BLADE

A Death-Dealer and a Day Walker teaming up to fight against bloodthirsty vampires? Sadly this hope was snuffed out too soon. Creators of the Underworld franchise approached Marvel Studios regarding a potential crossover with Blade but the company declined, wanting to handle the character on its own terms. During the Underworld: Blood Wars panel at New York Comic Con, Kate Beckinsale surprised fans by revealing the idea has crossed their minds.

However, much to the disappointment of eager fans, the actress hinted the people in charge of Blade were intent on keeping the character for themselves. Despite overall mixed reviews, The Blade franchise has received a cult-like following and in 2015, at San Diego Comic Con, Wesley Snipes teased about bringing the character back to theaters. Recently, Avengers: Infinity War actor Anthony Mackie has expressed wanting the play a version of the character in the MCU.

10 CROSSOVER WITH I, FRANKENSTEIN

In 2014, Lionsgate released I, Frankenstein based on the digital-only graphic novel by Kevin Grevioux. The film follows Adam, Frankenstein's monster, teaming up with a group of gargoyles created by the Archangel Michael to protect the human race from demons hellbent on taking over the world. Its overwhelmingly poor reviews cast doubt on any potential sequels.

Grevioux, despite the film's poor reception, expressed his intent to have a crossover film with the Underworld franchise. In the original version of his screenplay, Kate Beckinsale made a post-credits cameo appearance as Selene and various references to the Underworld universe were meant to be hidden throughout the film. Also hindering the idea of a crossover was the fact that both films share the same actors, including Bill Nighy, and Grevioux's fear that "mingling" the worlds would cause confusion.

9 BUSTING MYTHS AND FLOORBOARDS

Still considered one of the coolest getaway scenes in movie history, the first Underworld film features Selene, cornered by a group of Lycans, using a pair of Walther P99s to shoot her way through the floorboards to make her escape. The same stunt was performed in the 1992 cyber-punk film Nemesis. In a two-hour Mega Movie Myths Special of Mythbusters, the crew decided to test whether such a dramatic exit was plausible.

The first concern was the fact that flying debris and possible ricochets would pose a huge risk and hinder the person from making a proper escape. In the film, Beckinsale used only two 20-round clips to break through the floor. Tory, using a fully-automatic MP-5 and assisted by a 12-guage shotgun, failed to break through the support beams during the experiment and ended up firing all 360 rounds of ammunition.

8 TRIBUTE

Michael Sheen reprised his role as Lucian in the Underworld prequel, Rise of the Lycans which details the origin behind the centuries-old war between the vampires and werwolves. Raised as a slave to the vampire race, Lucian develops a romantic relationship with his master's daughter Sonja. Earning the respect of his fellow lycans, Lucian raises an army against the vampires, leading to an all-out pursuit of revenge when his lover is brutally killed.

During filming, Ron Howard paid a visit to have the actor record dialogue for Frost / Nixon. In the 2008 historical drama film, Sheen plays British journalist David Frost. Still covered in blood from a day of shooting, Sheen happily obliged and went to his trailer, asking someone to take a picture of him. The actor later had the picture sent to Howard who decided to frame it and have it displayed.

7 SETTING THE TONE

In Underworld: Evolution, the relationship between Selene and Michael becomes more romantic. In the first film, Selene helps him escape from the Lycans but he is bitten by Lucian, who reveals the reason for the feud between the two races and his desire to create a hybrid of the species. Seeing their growing attraction mirrors the love he once had, Lucian helps Selene and Michael uncover the conspiracy and aids them in killing Viktor.

After avoiding an attack from the newly-awoken Marcus, Michael shields Selene from the rising sun by driving her to an abandoned warehouse and darkening the windows with black paint. Once they are safe, the two share a passionate kiss, slowly undressing each other before making love. During a moment where the two look longingly at each other, Wiseman surprised everyone by blasting Bryan Adam's "Everything I Do" leading to the cast and crew bursting with laughter.

6 CONNECTIONS ACROSS TIME AND SPACE

Part of the success of the Underworld franchise is due to its talented cast. The same can be said of the BBC sci-fi series Doctor Who. Eagle-eyed fans of both may recognize a few familiar faces such as Bill Nighy, Tony Curran, and Derek Jacobi.

In the eleventh episode of Season 3 entitled "Utopia," the 10th Doctor and his companions encounter the mysterious Professor Yana, played by Jacobi, at the end of the universe. Little do they realize the elderly man is secretly the Doctor's greatest enemy, the Master, in disguise. After regaining his memory, he regenerates into a younger version of himself played by John Simm. In Season 5, the 11th Doctor aids Vincent van Gogh, played by Curran, fight off an alien only he can see. Bill Nighy has an uncredited appearance as a museum curator and devoted fan of the deeply misunderstood artist.

5 A RED REUNION

The fifth installment of the Underworld franchise, Blood Wars, features cast members from various seasons of Game of Thrones. The first is Charles Dance who played Lord Tywin Lannister, father of Cersei, Jaime and Tyrion. Known for his stoic yet cruel demeanor, he insists all that he does is for the betterment of his family despite having little affection for them. In Blood Wars, Dance plays Thomas, one of the leaders of the few remaining vampire covens.

Tobias Menzies, who played Edmure Tully and James Faulkner, who played Randyll Tarly also make appearances in the film. As Semira, Lara Pulver plays the ambitious leader of the Eastern Coven. In her desire for power, she attempts to become a hybrid, discovering a way to become immune to UV light. In the 2014 Game of Thrones: A Telltale Game Series, she voices Lady Elissa Forrestor.

4 TRACING THE BLOODLINE

The first Underworld film became the subject of a copyright infringement lawsuit by White Wolf Inc. and Nancy A. Collins in September 2003. The author claimed the plot of the film was based entirely on her short story "The Love of Monsters," published in 1994, which involves a forbidden romance between a vampire and werewolf.

Collins also went on to claim between 70-80 points of similarity between the popular film and White Wolf Inc's various role-playing games such as Vampire: The Masquerade and Werewolf: The Apocalypse, which is where the problem arose. Both games take place in a "fictionalized 'goth-punk' version of the modern world," which the Underworld universe has been compared to many times. The suit was privately settled outside of court for an undisclosed amount.

3 A SHAKESPEARIAN ROMANCE

While the main plot of the Underworld franchise centers around the centuries-old feud between vampires and werewolves, the original pitch for the first film was meant to be more romantic. Intended to be presented as a "Romeo and Juliet for vampires and werewolves," hints of what was supposed to be can still be found in the forbidden love between Selene and Michael.

Selene comes from the ancient race of vampires, trained as a Death-Dealer who hunts Lycans. Michael is a human descendent of the Corvinus bloodline who is bitten by the Lycan leader Lucien. Their paths cross when Selene discovers the Lycan's plot to make a hybrid of the two species. The two slowly unravel a plot to cover up the true reason for the war and in the end, Selene bites a dying Michael, turning him into the first successful werewolf-vampire hybrid.

2 UNDERWORLD: ENDLESS WAR

In 2011, under the supervision of Len Wiseman, an animated film called Underworld: Endless War was released along with three tie-in short stories that reveal events prior to Underworld: Awakening. Each story follows Selene, through three time periods, as she hunts down three powerful Lycan brothers. The film, directed by Juno Jun Lee, was done in an anime style.

In the first story, Selene is hunting the Lycans in 18th century Paris and successfully kills one of the brothers. Her missions continues in 1967 when the brothers crash the Crimson Moon Ceremony and kill one of the vampire lords. In 2012, she is reunited with Michael who aids her in putting a stop to the last Lycan brother. The pair also discovers humans have become aware of the existence of vampires and werwolves and have made technological advancements to deal with them.

1 SQUEEZING INTO THE ROLE

As the weapon-wielding, superhuman slayer Selene, Kate Beckinsale has had to undergo rigorous training for her role in the Underworld franchise. The actress admitted she had to "be taught how to run, how to throw a punch, all those things" in the first film as well as learn how to handle a gun and various medieval  weapons. Thought initially Blood Wars was meant to be her last appearance, it was revealed Beckinsale will be returning in the yet-to-be-named sixth instalment.

In various interviews, the actress states she's drawn to playing Selene because of the tragic nature of the character. However, the actress admits donning the latex catsuit hasn't been the easiest: "It's very squeaky all around," she revealed to The Sun, also saying she needs an extra set of hands to lace up and tighten the corset. Beckinsale also remarked the suit emphasizes a certain body part she's not a huge fan of: her backside.