Death is a Black Magic spell that inflicts instant K.O. on the target. It differs from the Doom status effect, which allows a short window of time to remove the effect before K.O. is inflicted. (In either case, standard EXP loss is still incurred for being K.O.'d by these effects.)
In most cases of monsters that use this spell, it is unresistable without having a Shadow Ring equipped. It cannot be blocked by "shadow images" such as those provided by Utsusemi or Blink, like most other offensive single-target magic can.
Death has a standard casting time of approximately 5 seconds. It is possible for certain enemies to cast it faster due to the influence of things like Fast Cast or Chainspell. Once Death begins casting, the only surefire way to avoid K.O. is to run away and attempt to get out of range of the spell, if possible. ("/assist <t>" might be a useful command to use on the enemy in determining who has hate in certain cases.) It may also be possible to interrupt the enemy casting it.
There are also certain abilities that carry an effect similar to Death, and again they are restricted to a very limited amount of enemies and unavailable for player use. Bhoot enemies can use Perdition which inflicts single-target Death, while a couple of Morbol NMs (Evil Oscar and Cirrate Christelle) can use Extremely Bad Breath which inflicts AoE Death.