Slay

Slay is a status effect that kills whichever creature it is applied to, unless they are explicitly immune to it.
Note that it is not the only status effect that kills a creature instantly -- other contenders are Disintegrate, Kill target and Power Word Kill. The party should be immune to these effects in Story mode.
==Gameplay==
As is true with all status effects, application details vary depending on source -- what is causing the effect. The source sometimes allows a saving throw or applies Slay only if the target is below a certain level or belongs to a certain species. Check the source you're interested in for such details. It is likely to help find ways to prevent Slay from being applied to party members, so they don't have to resist it explicitly using one of the means described in this article.
==General Prevention==
Resisting Slay can be done with the spell Death Ward or the high-level abilities Avoid Death (Rogue) and Favored of the Spirits (Shaman). Often times a Slay status effect allows a saving throw versus Death, so boosting saving throws can help. Some spell effects that include instant death may be defended against with magic resistance, such as Finger of Death. The Potion of Magic Shielding adds +20 bonus to all saving throws, so you essentially will (almost) certainly not miss a saving throw. See also some other prevention and their links in the Info box.