It shouldn't come as a surprise that a good number of villains are introduced in comic book events. Crossovers or major character events need an equally major antagonist for the heroes to overcome, so this is frequently the best place to introduce a powerful new threat. Sometimes these villains become major players that stick around for years after their introduction, while others wind up being one and done deals.
Source：https://www.cbr.com/dc-villains-event-introduced/
