These days, every year has a major crossover event series from Marvel. The company knows they will sell a ton of issues as fans will pick up every issue that ties into the main story and the main series with the battle. The most recent was Empyre in 2020, and the company is already gearing up for King in Black.
That doesn't even count X of Swords in the world of mutants. However, it didn't use to be this way, and the event crossover series were often only used sparingly and signified world-changing events. Today they are so common that the changes they make are rarely groundbreaking anymore.
10 Thanos War (1973-1974)
Before Marvel Universe's heroes battled Thanos for control of the Infinity Gauntlet, a crossover series titled "Thanos War" in 1973 occurred. Created by Jim Starlin, Mike Friedrich, and Steve Englehart. This event crossed over in books like Captain Marvel, Marvel Feature, and Avengers, with a total of 11 issues and featuring the first time that Thanos went into battle with the heroes of the Marvel Universe— specifically Captain Marvel.
9 Contest Of Champions (1982)
Nine years after "Thanos War," a new kind of crossover event series took place. In 1982, Mark Grunwald, Steven Grant, Bill Mantlo, and John Romita Jr. created the Contest of Champions. This played out across three issues and had the Grandmaster challenging Death to a contest of the Earth's champions. If Grandmaster won, Death would bring his brother, The Collector, back from the dead. Each side chose 12 players to battle in order to determine the winner.
8 Secret Wars (1984-1985)
Two years after Contest of Champions, Marvel shook up everything in its universe. Readers picked up issues and saw each of their favorite superhero comics end with the protagonists disappearing to some unknown realm. In the next issue, the heroes were back with major changes.
Examples were the fact that Spider-Man had a new costume and She-Hulk replaced Thing in the Fantastic Four. There were 12 issues in the Secret Wars base series, where heroes were forced to fight villains for The Beyonder's amusement.
7 Secret Wars II (1985-1986)
The year after Secret Wars, Marvel brought readers Secret Wars II. This time, instead of bringing the heroes to him, The Beyonder showed up on Earth. His arrival was in an attempt at enlightenment, but he ended up going into battle with Earth's heroes and villains.
This was Marvel's first mammoth crossover series. It stretched throughout 41 issues, with its own miniseries as well as stops in the pages of Fantastic Four, X-Men, The Aveners, Amazing Spider-Man, The New Mutants, The New Defenders, The Mighty Thor, Doctor Strange, and many more.
6 Evolutionary War (1988)
In 1988, Marvel took on a different tactic for its crossover series. Unlike the previous three, where major miniseries told the central story, "Evolutionary War" only played out in the pages of annuals to the monthly comics. This was a battle with the High Evolutionary working to adapt humanity to make it better than even the Beyonders.
This series played out across several annuals, including X-Factor, Punisher, Silver Surfer, X-Men, Amazing Spider-Man, and more. It concluded in Avengers Annual #17.
5 Inferno (1988-1989)
The same year that "Evolutionary War" played out across the Marvel Universe, there was also a mutant crossover series. This was "Inferno," and it saw Scott Summers' ex-wife Madelyne Pryor, become a major villain in the pages of X-Men.
"Inferno" also destroyed much of New York City, so it not only crossed over with all the mutant books but also series like Amazing Spider-Man, Avengers, Daredevil, Fantastic Four, and more. This was the first-ever company-wide mutant crossover series.
4 Acts Of Vengeance (1989-1990)
The next big Marvel crossover series was one where all the villains decided they should work together if they wanted to beat the heroes. This was titled "Acts of Vengeance" and saw Loki manipulating the big villains in Marvel (Dr. Doom, Red Skull, Kingpin, Mandarin, Wizard, and Magneto) into working together.
The major villains got minor villains to work for them and attacked. This was a massive event, with 69 different issues playing into the story, with just about every superhero comic affected by the attacks and the eventual outcome.
3 The Infinity Gauntlet (1991)
The first major crossover event of the '90s was the now-famous Infinity Gauntlet crossover event series. This is the book on which the movies were based, where Thanos got his Infinity Gauntlet and wiped out much of existence to try to win over the love of Death.
This was a bit different from the movies, as Mephisto played a role, as did Death, and the two most important heroes were Silver Surfer and Adam Warlock. There were six issued in the main series with 17 tie-in crossover issues.
2 The Infinity War (1992)
One year after Infinity Gauntlet, the follow-up came with The Infinity War. In this issue, Thanos was out of the picture, as he had decided he had enough. Instead, Adam Warlock had taken the gauntlet and then ended up becoming evil, with his evil side— Magus— becoming the main villain for the universe to battle. Like Infinity Gauntlet, this was also a six-issue series but had many more crossover issues, with 46 tie-in crossover issues.
1 The Infinity Crusade (1993)
The finale of the trilogy came in 1993 with The Infinity Crusade. This time around, Adam Warlock's good side, Goddess, brings together an army of heroes to fight in her holy war. It is up to Adam Warlock and Thanos to stop her from destroying all of existence. This was another six-issue miniseries with several tie-in crossovers. There were 38 tie-in issues, including Iron Man, Doctor Strange, Spider-Man, and many more.