A wizard of any level can, at any point, instantly, decide to use a death curse. This instantly consumes all memorized spells he has (CURRENTLY memorized) and deals damage described below. Alternatively, the wizard can make a specific thing happen to a single person (Like when Harry’s mom used her death curse to make the King of the White Court unable to feed), which would normally be done because the person in question cannot be affected by an attack of this nature.
The damage per spell level has no savings throw. It is affected by magical resistance in a % way. If you had 50% magic resistance then you would take half damage. This only takes effect if the wizard using his death curse against you choose for it to be an area effect. If he targets a single being, that being takes full damage unless it is immune to magical damage, in which case it takes none.
Death curses are SIGNIFICANT workings of magic, and the wizard can decide exactly what happens. The only limitation to this is the number of spell levels he has, which the DM should use to determine if his result is reasonable. For example: a wizard decides he wants to collapse the cavern by focusing his magic on a small part of the ceiling. That portion of the ceiling is obliterated, the cavern collapses. Damage should be reduced, but the people inside should be trapped or in deep shit.
Damage: The base damage is 1d6, per spell level consumed, in a 30 ft. area. If the wizard chooses, he can reduce the die rolled by 1 to add an extra 30 ft. to the area. As such, 1d5 for 60, 1d4 for 90, 1d3 for 120, etc. He can also decide to target a single creature for 1d10 per spell level.
No matter how a wizard uses his death curse, he is irrevocably dead afterwards. He cannot be resurrected. The only possible way for him to remain alive afterwards, is if he had done the work to become a lich, which he would then still do.