I've seen that sigil before. It's heavily familiar to me.
EDIT: Has Besiege been looked through yet?
EDIT 2: I first saw the sigil in the Vinesauce stream of Bellular Hexatosis. I must remember it so clearly because I watched it live, and Vinny received an e-mail complaining about how he had completely missed the meaning of the game and dismissing it as just "weird".