When Keine Kamishirasawa from the Touhou games transforms on the full moon, goes crazy, and rapes you in the ass with her horns.
The term's association with said ass-raping comes from a butchered English-to-Japanese machine translation where "caved!!!!" became "screwed!!!!". Used by mostly Japanese-speaking Touhou fans, but a few English-speakers have also adopted the term.
I was caved!!!! by EX-Keine last week, and I STILL can't sit down!
An affectionate nickname for the character Van Grants from the PS2 video game Tales of the Abyss. Was chosen due to the creepy pedophile aura surrounding him, and his very close and manipulative (but not canonically sexual) relationship with the underaged hero, Luke fon Fabre.
In addition to touching little boys, Master Badtouch can also shoot lasers from his eyebrows.