Played by Japanese priests in the 16th century, taikyoku shogi may be the largest variant of chess ever devised. Each player deploys 402 pieces of 209 types on a board of 1,296 squares to try to capture his opponent’s king(s) and prince(s).

It’s not clear precisely how it was played, but Wikipedia takes more than 10,000 words to describe one likely set of rules.

(Thanks, Alejandro.)