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STDEATH.ITM is an undroppable melee weapon used by a specific version of Arkanis Gath.

It is considered an enchanted item and deals 1d6+3 Piercing damage (adjusted by Strength bonus), with a range of fifteen feet, and speed factor of 0.

The weapon functions as a +3 Short Sword, however each successful strike on a target exposes the hit creature to a Slay effect. This effect has no Saving throw and can't be blocked by Magic resistance or spell protections, such as Globe of Invulnerability or Spell Trap.

A Death Ward can prevent the killing effect. Stoneskin will negate the damage but the target will still be exposed to the Slay.