This works on all summoned creatures, and is particularly effective when used with Shambling Horrors.
Perhaps even more effective is when used with the spirit Anguish as when it hits a hexed foe the bonus damage is also doubled. This works well with Painful Bond.
Reapplying Signet of Ghostly Might will not reset the death counter.
During the Preview Events this skill was known as Signet of Might.
In its original form, this skill was bugged, and could be cast on any target, friend or foe (players, monsters, NPCs: including Guild Lords) and was exploited during PvP matches. There was no way to remove it, and 10 seconds later, the target would die.