A rune that, when used on a rare item, will empower that item in the same manner that rare items are upgraded after empowering a nemesis encounter twice.
This rune should only be usable on rare items that have not had any crafting done on them. This could help keep rare items relevant, even in the late game. Found a nice T20 rare item with some desired affixes on it? Slap a Rune of Vengeance on it to maybe get something amazing.
Rune of Vengeance could be a rare drop from nemesis encounters, or just a random drop.