This evil bastard loser is Scorned Rhythm, a failed singer/guitarist who was ousted after stealing credit for the lyrics his music partner wrote
Eventually he found himself a broken chunk of a necromancy artifact, and stole it for himself
It’s incomplete, so needs more magic than usual to use, and being a shitty spellcaster means Rhythm needs additional concentration to make it work - this, of course, comes in the form of singing
Because he’s a fucking idiot, he just straight up thinks that you need to sing in order to use the amulet, which isn’t really the case
Despite the roundabout methodology, he has managed to make it work, and is getting better and better at summoning ghosts and undead to wreak havoc, while he goes around stealing things in the ensuing panic
He’s resurrected three ghosts he uses as backup singers for his phony performances