When a PC is stat-drained by a monster's melee attack and the corresponding stat goes to 0 or less, PC automatically dies and gets a post-mortem message "is transformed into a -foo-".
This works nicely for wraiths, wights, vampires, spectres, shadows, slow shadows and shadow lords.

However, living walls (constructs), quasits (demons), shadow centipedes (undead insect, not spirits) and shadow trolls (undead trolls, not spirits) can't really transform the PC into one of them (confirmed death for all these monsters).