The last part is optional.
It's obviously a fairly high level spell, and is only going to be available as a ritual casting, the preparation and actual casting taking at least an hour. I just need guidance as to the level of the spell, proportion of the spell power returned to the caster, and costs/materials (aside from the usual necromantic nastiness of blood sacrifice and playing The Jackson Five backwards on 33 RPM or whatever the fantasy equivalent is of that.)
Please advise!
It's a very stereotypical BBEG Necromancy spell that
•Casts Desecrate over the spell's area, bolstering undead.
•Absorbs the life force of creatures that die in the spell's area (which then can't be brought back short of a True Resurrection spell) and returns some portion of that energy to the caster as restored spell levels.
•Uses the other portion of the absorbed life force for another goal, like a portal, an ancient artifact that needs a "jumpstart," or a magical doomsday device.