It might just be my understanding but we have seen life and soul drain being necrotic, demonic and eldritch.
Nihil in shadowmoon dungeon WoD is there eating souls and shadow priests have vampiric touch.
Frostmourne eats souls and death knights have a self healing attack (death strike I think?) and there is other soul things in necromancy, from liches, the naxxramas two room boss and MoP scholo rattlegore. And of course the reliquary Of souls in ICC dungeon.
WC3 dark rangers have the life drain.
Fel has as stated soul engines. And guldan uses life drain to age orc children.