@Abenpaap, Once significant numbers of people think that Erebus is Shadowfel, there's no way for someone to prove it one way or the other with information from within Erebus. The only way for a definitive answer is to look at the position of the world the characters are in relative to other worlds. I.e. Only those who've mastered Dimensional magic can give a certain answer, all other answers are based on faith.
This provides a reason for Weyoun to push for research into necromancy. Once the research is complete, Weyoun and his associates can claim their monopoly on the truth and announce authoritatively whether they're in Erebus or hell. If he says it's hell, no one else is really in a position to challenge it (except with, "We'll never believe you, no matter what evidence you say you have."). If he does this, the stage should be well set for the Pact of Ragnarok, or Ashen Veil.
That's a good point; users of Dimensional magic could disprove the lie or confirm it. If the greatest users of Dimensional magic in the world claim they are in Esus' vault, that would make almost everyone believe the lie, except those who either take it on faith (as priests of some other gods will likely be claiming they are in Erebus) or those who actually realise Ceridwen is tricking people. I suppose they would begin spreading the story that any way out will be a lie, and that the only way to truly get out would be to destroy the vault around the same time.
By the way, when are you planning to begin spreading the lie over Erebus? I'm looking forward to it, hehe.
(By the way, it's 'apenpaap', without capitals and with a p )