Irhan's Rage

Irhan's Rage is a sword able to kill and defeat any Wonders. It can be wield only by a Wonder

We had this sword since the beginning of our adventure, being the sword who sealed The Undying and was in our Portable Hole.

For the case of The Undying, according to the records found in Vizerrik by the party who sealed her:

If the blade were to be wielded by one of Wonderous blood under a full moon, then it could slay the Undying.