"So what exactly is SH Avalon, and what's it about?"
SH Avalon is a new game mode inspired by Avalon: The Resistance for secrethitler.io that introduces 3 different roles to the game of Secret Hitler.
"Cool, so if I'm Merlin, what's stopping me from just claiming all my information from the get go?"
Because! Fascists now have a new win condition. If liberals reach either of their objectives - either via executing Hitler, or by getting 5 liberal policies on the board - then Hitler is given a chance to steal their win getting to select one person to assassinate. (The fascists get revealed to Hitler during assassination.) If they assassinate Merlin, then fascists win instead. So be careful as Merlin to not give out too much info!
Important Clarification: Merlin getting shot only results in a Fascist win if he is shot during the post-game assassination, NOT if he is shot via the bullet that came from the 4th and 5th fascist policy being enacted. If he is shot as a result of the latter, the game continues on as normal, and no mention is made whether the deceased player was Merlin or not.