KILLFUCK SOULSHITTER is a nickname for any crazily overpowered magical attack in an RPG, either online or off. It dosen't have to be the game's most powerfull spell- just sufficiently powerful enough to ensure its target is totally screwed and without even a remote chance of survival. KILLFUCK SOULSHITTER usually involves fireballs or explosions, or both. I'm fairly sure I heard this used somewhere before, but I don't remember where.
A: Yo man, check out this spell.
B: Yeah, what's it called?
A: KILLFUCK SOULSHITTER
*nearby town incinerates*
A generic name for ridiculously ultraviolent barbarian-type characters who exist only to murder and destroy and crush, especially in tabletop RPGs (where designing a character like that can be mechanically powerful, especially if the system penalizes you for having emotional attachments.)
The term comes from RPG.net, where it was originally mentioned in an ancient thread of horror-stories about terrible character names people have encountered. Often paired with Snake Gandhi, a similarly ridiculous name from that thread.
"So, yeah, the only weakness to this defense is if you're faced by someone you love... but if you play Killfuck Soulshitter, whose only intimacies are to Killfucking and Soulshitting, you're basically invulnerable."