Here is a comprehensive summary of the Wikipedia article on Baroque chess:
Baroque chess, also known as Ultima (a name change suggested in 1963), is a chess variant invented by Robert Abbott in 1962. Played on a standard 8x8 chessboard, it distinguishes itself from traditional chess by requiring players to decide on initial piece symmetries, such as the placement of the King and Withdrawer, or designating "rooks" as Coordinators or Immobilizers. While the King moves and captures like a standard chess king, the objective being its capture, all other pieces have unique movements and employ more complex capturing methods. For example, pawns move like rooks and capture by "squeezing" an enemy piece between themselves and another friendly piece. Despite Abbott later suggesting rule amendments, the gaming community largely adheres to the original 1962 rules, leading to some regional variations of the game.