From publisher blurb:
Did you enjoy the undead options in Old School and Cool 4?
Ars Moriendi has more undead, more Invultuations, more vulnerabilities, more tables, upgraded undead, adventure seeds, and in general more bleak, decaying, grody, rotting good times.
Play as revenants, mound dwellers, dullahan, wights, spectres, or gloam dancers.
More Invultuations such as Turn into Hyena, ghoul bite, or summon vermin.
More cursed items like the banshee's veil, or the soulreaver's scythe.
Undead Companions like the bloody skeleton, the dhampir, or the nekomata.
10 more vulnerabilities, including that classic of counting millet seeds.
Suggested adventure seeds and ultimate undead upgrades.
Lots more!
You need Old School and Cool volume 4, and thus Old School Essentials or another old school ruleset to use this book.