Vanilla is a product of Lussumo:
Documentation and Support.

" all the game’s content is winner-generated. That means that the game starts out barren. One class for players to play, one monster to fight, and one item to use. But every round, the top two scoring players use a tool built into the game to make a monster, item or class and then that object is automatically inserted into everyone’s game, and players fight for new high scores in an entirely different game every round."