Spawn
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I was never a big fan of Spawn, but at the time when Todd McFarlane's creepy creation debuted, I didn't miss the 'hugeness' of it all. I did pick up Spawn #1, partly out of collector's instinct, and partly out of creative curiosity.
McFarlane's run on Spider-Man had been a big hit, but looking through the pages of that Spawn book, I knew I had never seen anything quite like it. Before you knew it, the comics, toys, the HBO cartoon and the movie had hit.
Anyway, the whole enthusiasm for Spawn definitely cooled, and I don't think Spawn has ever became what I'd call "mainstream", but the phenomenon will always be remembered by many as a bookmark of the 90's.