An event (also known as a "crossover event" or "mega-event") refers to a story with a much bigger reach than a regular story arc or crossover since its main narrative (which starts and ends in a core mini-series) bleeds into other comics.
Comic issues with stories adjacent to the main narrative are called tie-ins. While the stories of the tie-ins revolve around the event's main narrative, they're only complementary and it's not necessary to read them to grasp the main story. There are different types of tie-ins. They can constitute of a mini-series or one-shot created exclusively for the event, or one or more issues of an ongoing series. The cover of a tie-in will be marked to distinguish it as such. In most cases, the layout of the cover will either match up the distinct design used by the covers of the main mini-series, or make use of elements reminiscent of it.
The word "event" itself describes the type of stories associated with them. Due to their nature, events require many characters to be affected by the main narrative in some way or another. Since events are massive in scope from a publication perspective, the case is the same from a perspective within the story. Events feature developments of great importance meant to cause repercussions, like world-endangering threats or big shifts in the status quo of the landscape of the superhuman community. It's not uncommon for events to also feature the death of characters in order to spark interest and reinforce the stakes of the story.
An example of an event is Civil War, a story which pits Iron Man and Captain America in an ideological battle over the Superhuman Registration Act. The main conflict of the book was told in a 7-issue mini-series called Civil War. Ongoing series like Iron Man and Captain America had tie-in issues that fleshed out each character's personal narrative, while other series like Amazing Spier-Man and Fantastic Four used tie-ins to highlight their protagonists' role in the conflict since they were secondary characters in the main book. Wolverine tied into the event, but since Wolverine himself was not present in the main conflict, his series told an adjacent story that stemmed from it. While Uncanny X-Men didn't tie into Civil War, a mini-series named Civil War: X-Men created for the event did. Another of Civil War's companion mini-series was Civil War: Front Line.
Some of Marvel's most noteworthy events include:
- Secret Wars II
- Infinity Gauntlet
- House of M
- Civil War
- Secret Invasion
- Avengers vs. X-Men
- Secret Wars
- Secret Empire