Q: Can someone reveal a Creeper hidden under their Elder Sign after the game has ended due to an UnGoal, to secure the victory via Cult Clash?

A: There are two situations which could be the case. The first one is actually very easily resolved, but the second one is a bit more complicated.

1: Doom points are even, and the revealed Creeper would tip the Doom points in one player’s favor.
2: Doom points are uneven, and the balance would change when the hidden Creeper is revealed.

1: Since Cult Clash is in play, there can be no situation where all players lose. Cult Clash is intended to ensure there will always be a winner (for example, in a tournament situation, you’d want to have a winner for every game). Therefore, in this case, if both players were tied for the number of Doom points, you do as you would for any “tie” or “simultaneous win”: the game continues until there is a clear winner. So the game has not “ended due to an UnGoal,” and the player with the Elder Sign simply reveals their hidden Creeper, and wins.

2: If, on the other hand, the game ends due to an UnGoal with Cult Clash in play, and Player A appears to have the most Doom points, until Player B reveals a Creeper hidden under their Elder Sign, then Player B is essentially stealing the win from Player A, similar to using a Surprise to cancel a game-winning play, so just like using a Surprise in such a situation, it’s all going to depend on INTENT and TIMING.

If you’re looking to cancel someone else’s win, you’ve got to be really on-the-ball about it. Basically, Player B, with the hidden Doom Points under their Elder Sign, needs to be prepared for the possibility of the Cult Clash victory so that they can deliberately reveal their hidden Creeper to steal the win AS SOON AS THE UNGOAL IS TRIGGERED.

If Player B only realizes they have more Doom than they thought AFTER the count has publicly been made… they missed that opportunity. Cthulhu called, and they were out to lunch or something.