The power of forgetting

Just a little idea I had for a campaign I'm running:
The outer planes are built on belief. Belief made them, creates them, keeps them alive and, well, at least a little real. We all know that, if you gather enough believers around you, the multiverse may change in your favor. Maybe a little, maybe you can shift whole layers from one plane to another.
But this also has a downside. If enough people stop believing in something, will it cease to be? What if we all forgot about something and it just stopped existing? Interesting thought, isn't it? Far from anything possible, you may think, how would enough people forget something important enough to be worth the effort?
Wrong. People forget all the time. They forget dates. Names. Faces. They forget the past. They forget to think about the future. They claim that they don't forget the important parts, but don't petitioners eventually forget everything of their previous lifes? Don't the gods themselves drift to the astral when no one bothers about them anymore?
I tell you, partner, with just a little... start-up, initiative, if you want it, people could forget almost anything. And I think I have found the little thing to make this possible...
Long story short: What if it were possible to make the multiverse forget something and it stopped existing?
Enter Krisuvian, a signer. For a long time he believed that he created his own world, that what he believed strongly enough would come true. Years passed, and he became dissappointed. Of course, the sign of One had certain possibilities, but that was the combined power of his faction, not his power as an individual.
He became power hungry, ambitious. He searched ancient texts in far away libraries. Talked to ancient and dangerous things. And then he found it. A powerful artefact, hidden away in the deepest caves of Pandemonium. The Sign of Oblivion. A crew of Adventurers, all soon gone mad from their search, found it for him, brought it too him.
It made possible what he had desired. Not all of it, but part of it. By forcing himself to forget something, he could erase it from the minds of the multiverse.
Of course, the Sign could not control all minds. Beings of power would always resist. Powerful mages, abyssal lords, powers, the lady and many others would remember. It didn't matter. Countless people would just forget and so believe would fade, and with it the objects of that belief.
He began experimenting and realized one terrible flaw in his plan: His willpower was not strong enough to activate the sign.
He gathered others like him, founded a cult. Finally, when their numbers had grown to several dozens, they succeeded. Their success was small, they had removed an old and barmy bubber from the streets of Sigil and it recuired a huge ritual, going on for hours, all of them in full concentration, concentrated on forgetting. They had to perform the ritual in the hollow cave of a dead god, a powerful symbol of the lack of belief.
But it was a success. They had removed a creature from the face of the multiverse, and no one remembered, not even his family. What could stop them now?

So. This is the situation I will put my players up against. Note that this artefact still recuires the right time, place and ritual, as well as dozens of people of exceptional willpower, it cannot be used at will, all the time. Perhaps some other foci of their power is needed.
So, my questions:
Is this a good idea or is it too powerful/epic/unbalanced/out of canon?
What else would the cultists need to perform this ritual? (basically, what could the players do to stop them)
I guess almost anyone in the multiverse would try to stop them. Various factions. Anubis, because it concerns dead gods. The Inevitables, perhaps, as making an intelligent being just disappear from the face of the multiverse is a horrible break of many laws. Who else?
Also, any other comments?