This game uses a modified Fairy-Stockfish engine and ffish rules library, and the Chessground board from Lichess. Their copyright notices and GNU GPL licenses are included with the source.
The chess app source is available under the GNU GPL version 3 or later. Engine changes include hero abilities, Monster +2/+3, Scottish Progressive, and multi-action search. The engine archive matches the source checksums recorded for the browser binaries.
Upstream: Fairy-Stockfish · Chessground