Slowdown - -50% initiative.
Plague - duration 6 turns
Disrupting Ray - -7 defense
Grief - -5 luck/morale
Weakness - -15 attack
Blindness - 0.3*sp
Berserk - the simplest option is to increase damage, or a more complex one: from level 0 in Dark Magic to Expert Dark Magic, simply increase the damage, and only extend the duration by two turns at the Extra Expert level (it's still better to just increase the damage)
Subjugate - 0.3*sp
Vampirism - depending on the skill level, the base percentage of absorbed damage is 10%, 20%, 30%, 40%, 50%
Undead - for example, 150+15*sp

Probably, it will be too overpowered, I need to cut it down:smile47:

Added 7 minutes later
Yeah, I probably overdid it:D - I need to make it a little less powerful

Added 1 minute later
For example, like this - let the Extra Expert level only affect spells of the first three levels