Kill target

Kill target is a status effect that kills whichever creature it is applied to, unless they are explicitly immune to it. Vorpal weapons rely on this effect to sever an opponent's head in a single blow. Definition of Vorpal in the Forgotten Realms wiki
Note: It is not the only status effect that kills a creature instantly -- other contenders are Disintegrate, Power Word Kill and Slay. The party should be immune to them 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, applies Kill target only to Undead creatures, or through a weapon on-hit effect. Check the source you're interested in for such details. It is likely to help find ways to prevent Kill target from being applied to party members, so they don't have to resist it explicitly using one of the means described in this article.
In addition to the death effect on the creature, the Kill target status effect provides several "Death Type' values that can be coded into the item, spell, or ability that is causing it. the status effect also allows a Text notification. See below:
Instantly kills the targeted creature(s), in the style specified by the 'Death Type' value.
Known values for 'Text Notification' are:
0 Show death message
1 Don't show death message
Known values for 'Death Type' are:
0 Acid death
1 Burning death
2 Crushed death
4 Normal death
8 Chunked death
16 Stoned death
32 Freezing death
64 Exploding stoned death
128 Exploding freezing death
256 Electrified death 
512 Disintegration death
Some of these death types will permanently destroy the creature, as well as carried loot (except items flagged as 'Critical'). Party members that are killed in the Death Types of chunked or exploding are typically not revivable via Harper's Call, Raise Dead or Resurrection type magics.
==General Prevention==
Resisting Opcode 13 from a player's standpoint can be done with the spell Death Ward. Often times an Opcode 13 instant death 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, and some may not, such as Vorpal Hits from a Planetar's Weapon. 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.