I'm looking for an RPG where necromancy, demon summoning, blood magic, and the like are the main focus of gameplay. If needed I can homebrew something up using the Storyteller system but that's a last resort.
>>54191799
Ars Magica is based around wizards of various orders including hte ones you mentioned.
Try Sorcerer. It's a game centered around demon summoning, hubris, and figuring out what you're willing to pay for power.