The Avengers are Earth's Greatest Heroes, and their villains have to be impressive to match them. The team has faced down nearly every kind of threat imaginable, but not all of them are equal. Some Avengers' foes are scarier than others, their complete lack of morality, body counts, amazing power, or success against the team makes them frightening to fight.
These are the villains that keep the team up at night, the ones who commit acts that cost countless lives and endanger the most powerful and skilled heroes on the planet. The Avengers take these villains seriously, as they can turn the tables on the team in a heartbeat.
10 The Grim Reaper Will Kill Anyone Who Gets In His Way
The Grim Reaper hates his brother Wonder Man and won't let anything stop him from getting revenge. Trading his hand for an energy-firing scythe, the Grim Reaper has no qualms about killing anyone, and even death hasn't stopped him. He's mastered necromantic magics that keep him coming back, making him a constant thorn in the team's side.
The Grim Reaper isn't a world-ending threat, just a villain who will kill anyone in his path. Life has no meaning for him, so he has no issue ending it. Even when it seems like he'll never be seen again, Grim Reaper always shows back up for another go.
9 The Apocalypse Twins Were Able To Outsmart The Avengers
Uriel and Eimin are the children of Horsemen of Apocalypse Pestilence and Archangel. Stolen by Kang and raised in a mutant concentration camp in the future to toughen them up, the Apocalypse Twins returned to the present day with a plan to save mutantkind at the cost of humanity. They played the Uncanny Avengers against each other and succeeded in getting the Celestials to destroy the Earth.
While they were eventually defeated, the Apocalypse Twins made the Avengers look like chumps, manipulated the Celestials into doing their bidding, and destroyed the Earth. Their time-based powers and Celestial armor make them devastating in battle, rounding them out as nearly perfect villains.
8 When Thanos SHows Up, The Avengers Get Nervous
Thanos is more of an overall Marvel villain than one of any particular team, but the Avengers have had to deal with the Mad Titan more than any other team on Earth. They battled the Mad Titan when he got the Cosmic Cube, members of the team were part of the hero task force meant to distract Thanos when he had the Infinity Gauntlet, and the Avengers defeated him when he came to Earth to kill his Inhuman son Thane.
Thanos's power and nihilism make him the most dangerous villain in the universe. There's nothing he loves more than destruction, and the mere mention of his name can send paroxysms of dread through the stoutest hearts.
7 Baron Zemo Challenges The Team Like Few Others
The Masters of Evil are the Avengers' powerful opposite number. They've battled the team for years but truly reached the next level of danger when Baron Helmut Zemo took over the team. Son of Heinrich Zemo, Helmut's tactical mastery and desire for revenge allowed the Masters to get their most decisive wins against the Avengers.
Zemo may just be a normal human, but his schemes make him a frightening opponent. It's impossible to know just what Zemo has up his sleeve until it's too late. He's done damage to the team that other villains can only dream about.
6 Loki Is The Avengers' First Villain
Loki has mellowed in recent years, but that doesn't change just how dangerous of a villain he can be. Loki is known for wanton cruelty with his callousness driving him on. The trickster god's magic is his most frightening power, but his strength and hand to hand skills are nothing to sneeze at . Loki is a threat on multiple levels and can challenge the team like few others.
Loki has spent millennia coming up with tricks and traps of all kinds. What makes Loki so frightening is that no one knows what direction he'll go in when he shows up. He may be a merry trickster, a brutal monster, or anything in between.
5 Korvac Has Killed The Team
The Avengers face off against the most dangerous villains of the Marvel Universe, and among those foes, Korvac takes the cake. The cosmic-powered villain laid waste to the team the first time he faced them. Korvac's nigh-omnipotent energies allowed him the kill the majority of the Avengers. The only reason they were ever able to beat him, in the long run, is because he resurrected them.
That's what makes Korvac so scary. He can kill the Avengers at any time, and there's really nothing they can do about it. Korvac has the power to decimate the team and exactly no qualms about doing so, making him a special kind of foe.
4 Red Skull Is A Monster
The Red Skull loves to be evil. The Nazi mastermind has battled Captain America for years, and this has made him a foe of the Avengers as well, attacking the team multiple times. The Red Skull doesn't usually have powers, although he once boasted Charles Xavier's mental abilities by grafting the dead mutant's brain to himself, but that makes him no less frightening.
The Red Skull is scary because he doesn't care who dies as long as he succeeds. His schemes when fighting the Avengers revolve around terror attacks that can lead to massive loss of life. He's an inhuman monster, one whose twisted visage says everything it needs to about who he is.
3 The Void Is An Extinction Level Event
The Sentry was the most powerful member of the New Avengers. He was also literally his own worst enemy. The Sentry had a second personality, the Void, which had access to all of his powers. For years, the two beings fought a fruitless war against each other, with the other heroes eventually wiping the Sentry from everyone's memories to keep the Void at bay.
Sentry's return as a superhero meant the Void could come back, and the inevitable happened. Norman Osborn used the Void as his greatest weapon, almost destroying Asgard with its power. The Avengers have only ever defeated the Void because they were lucky, as its power was greater than they could imagine.
2 Kang Has Conquered The Future
Kang is an iconic villain. The Conqueror earned that appellation because he defeated every foe of the future and set his sights on the Age of Marvels' greatest heroes. Using far-future technology and his amazing tactical abilities, Kang has challenged the Avengers like few others.
Kang has spent years traveling through time and space, perfecting ways to kill his foes. Luckily for them, Kang is an honorable man, or he could murder them all in their cribs. That's what makes Kang so scary; the only thing that allows the Avengers to continue to exist is his sense of honor, which could disappear at any time.
1 Ultron Has Killed Millions
Ultron is the Avengers' deadliest foe. The creation of Hank Pym, the evil android desires nothing more than the death of all organic lifeforms and has killed millions of people over the years. His weapon systems, strength, and killing ability are second to none. What truly makes him dangerous is his adamantium shell, allowing him to take attacks that would end any other enemy.
Ultron is an inhuman killing machine created by the Avengers. Every life that he takes is the team's fault in some way, and he's very good at killing. Ultron's return sends chills through the Avengers, as the body count is always sure to be massive.