A new one, it's unbearably op but I think it's kind of cool:
This Word cannot be represented by letters, rather it appears as, and somehow is heard as, an image of a holy lotus flower, dead and crumbling. Whatever it is used on has its being stripped from it, becoming nothing more than a faint, meaningless memory in the Akashic Library, doomed to decay into nothing. As the DM, stop mentioning the target in any way, whether through narration or npcs, and instruct your players to do the same, as the target no longer is and never was. This word is the most feared of all powers, and the Supreme Being, unable to successfully seal it or destroy it, has locked it in the most dangerous part of the Akashic Library and purged all knowledge of it, and indeed, the very capability to comprehend it, from creation.
The inspiration here is that the word strips off any and all qualities of a being, attacking the very ideas behind the character or location or what-have-you, causing the "Akashic Library," here representing the gaming group, to "lose its grip" on the thing, rendering any interaction with it impossible, until everyone collectively accepts it, moves on, and someday forgets it existed. I came up with this as a possible "ultimate attack" of sorts for Heka, my settings embodiment of magic. Not just as in the thing wizards do, but all things fantastic and wondrous, from brave adventurers fighting unusually large rodents to cosmic beings controlling the laws of physics like Play-Doh.