This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Accel World
The Disaster Armor. Not only does it have the appearance of a monster, but when it's mouth opens, it reveals what looks like a black hole. And then there's it's sole instinct: to devour anything and everything.
The Black Hole has the same art style of the Incarnate System... at the least, when Noumi starts his for the first time in the series. Given that the Disaster Armor is first created through abuse of the Incarnate System... it appears Haru nearly revived the Disaster Armor at one point. Thankfully, the time ran out.