A MEGAGAME is like a 50 player board game, but with typically a LOT of forced social interaction.
Megagames typically do not have a clear-cut winner or loser. Megagames are typically more of an interactive storytelling experience where dozens of players get together in a single location at a single time to “play” this game/experience. These get togethers are typically held face-to-face, but with recent events, more efforts are being made to convert existing and make new MG’s to be Online-Play enabled.
UberSpace Legacy (USL) is a BLEND of a Megagame, a Board Game and an RPG Role Playing game.
Check out this video to get a good idea: