Dark Sacrifice Rank 1
Dark Sacrifice (Rank 1) is a Priest spell in the WoW Forever client.
Cannibalize 400 of your own Health over 15 sec to gain 400 (+100% of Spirit) Mana.
Learned from
Learned from the quest Devouring Plague at level 20, not from the Priest trainer. The trainer sells the higher ranks. Speculation: this is Classic's class quest; its reward spell is in the Forever client, but nobody has confirmed the quest in Forever yet.
Data from the WoW Forever client, build 1.60.1.70862, extracted .