Lately I have added "death attacks" to a few mobs (the banshee's wail and the beholder's black ray, for example) using a game mechanism that I saw in D&D and liked. I thought I would explain how it worked.
Basically these attacks roll damage like a normal attack, possibly quite a lot of damage. If that's not enough damage to kill you, then nothing happens (you don't take the damage). If it is, then you die.
In general I will try to make any further death effects work like this.