The Brief: Merc'd up is a video game slang term meaning messed up, destroyed, or killed.

DEEP DIVE
Merc is a derivation of the word mercenary. In video games, merc, mercing, or merc’d up usually means to kill a character or otherwise pwn someone in a game. The word can also be spelled murc, murk, or merk.
Had a dream where I was in the walking dead and my whole squad got mercd π
— AdΓ‘n Lozoya (@LozoyaDaft) August 28, 2019