Nighmare CurseThe dreaded Shobastian Nightmare Curse causes your enemies to fall into a blissful slumber. Meanwhile, their brains are beset by Eldritch Things who cunningly replace their internal thinkmeats with the Bees of Evil. Caution: exposure to Bees of Evil may prove to have toxic side effects to the caster.
Deathly Hex: The debuffs applied (reduced health regeneration for ~90 turns, and reduced health regeneration for 6 turns) do not visibly stack, yet you still take 2-3 damage per round for a few rounds after casting. This damage stacks, and can very easily kill your character.
Nightmare Curse: This will apply a damage over time effect to the enemy that scales (I believe) with magic power. It is extremely damaging, often killing a full health monster over 3 ticks. Additionally, it will usually put the monster to sleep. The downside is reducing maximum mana by 2 for 70+ turns, and reduced mana regeneration for about 5 turns.
Tenebrous Rift: Creates a large area of effect that takes damage over time for 10 turns. The center is a 3x3 area, and it also seems to damage 1-3 squares beyond this. Sometimes the caster is cursed with a DOT as well, but not the majority of the time. Save your game before using this spell for the first time, and cast it at maximum range from yourself.
All of these spells have a negative side effect after casting. These will typically end up with you taking damage every turn, having reduced (even negative) mana regeneration, and reduced stats. These spells are for advanced players only.
The necromantic resistance required to take no damage from the effects of these spells depends on your magical power: The formula is 1 + (magic power/10) rounded down. So 55 magic power requires 1 + 55/10 = 1 + 5 = 6 necromantic resistance.