Seeds of the World Tree

世界樹の種 · Magic Items · World Items

A World Item that can change one's race (side story The Vampire Princess of the Lost Country).

Race-change items exist outside World Items too, but in the game this is the only one that can turn an undead into another race
From the side story The Vampire Princess of the Lost Country (not main-story canon)
In YGGDRASIL, once your race is changed to undead, ordinary race-change items no longer work; the only options are Seeds of the World Tree or deleting and remaking the character (side story-074)
In the side story Suzuki Satoru also wondered whether the super-tier spell [Wish Upon a Star] could substitute for it, but never tested it

Notes

Side-story setting; whether it exists in the main story, and who holds it, is undisclosed.