Kill trigger

The entity is called trigger_hurt - they are created by texturing a brush with tools/toolstrigger and using ctrl+t to tie it to that entity type. In its properties, set the damage to something super high like 9999 and choose the appropriate damage type to display in the kill feed.

Also set "zero damage force" to yes, if you want players to die by falling in a pit, so the ragdolls won't look like they got hit by an invisible flying train.