This Sacred Oath — Meg Hendry

Halbert makes a deal with Mortua, Goddess of Death, in an attempt to save his wife. The bargain takes an unexpected turn. Halbert must find a way to free himself of the oath, or serve the goddess forever. Becoming an undead necromancer seems to be his only way out. Along the way he meets Norris, who has also sought out necromancy, but for a very different reason. Halbert is the only one who knows Norris's plans, the only one who can stop him. This Sacred Oath is a short story.