*UNCONFIRMED*
Second hand information, but I witnessed the phenomenon within Freemarch but was unable to determine if this is what caused it at first. They did brag about it in zone chat tho.
Presently there is a quest from the world event to "Kill invading forces with a [Death Stone]"
This quest item is not supposed to have the ability to kill elites, but it will pop them instantaneously when used on them.
The quest is limited to 5 invading creatures (including guardians/defiants) and is a daily quest.
Areas to explore:
*Use of the item within PVP
*Use of the item within WF PVP *Confirmed not working*
*Use of the item within on an opposing faction Town/Wardstone
Edit:
Additional Information
Sub-elites (Twin swords) can be destroyed with this item
Quest is called "With Death Denied"