Standard chess setup, except that there are no chess pieces on the first and last rank. The rest of the pieces appear on the board during the game.
The movement of pieces and their 'reproduction'. Chess pieces move across the board as they do in ordinary chess - according to the standard rules of move and capture. After a piece leaves the square on which it stood, a new chess piece appears on this square, according to the following chain of cycles: Pawn - Knight - Bishop - Rook - Queen - King.Thus, after the pawn's move, a knight appears on the square where this pawn stood. On the square where the knight stood, after its movement, the bishop appears, and so on until the king appears. So one piece generates (or spawns, idk which word is better) another, and only the king does not generate any pieces during its movement. With each move, the fields of the board fill up, turning a chess battle into an unprecedented opposition of chess pieces.
The advantage of the first move in the 'Battle of the Kings' does not matter at all, since by making the first move you trigger a self-destruction mechanism - sooner or later all your pieces will evolve, filling the surrounding space, and a collapse will occur. If you let your opponent do it first, then you have won. The paradox of the game is that, on the one hand, you have to fight for living space, constantly increasing your strength and power, and on the other hand, do not become a victim of your own strength and power.
It is not possible to calculate anything in this game. We can be guided by intuition and instincts, which tell us that the appearance of kings should be postponed as late as possible, and hence the appearance of queens and rooks is not desirable. However, for each move a new piece appears on the board, and the appearance of new pieces, and more senior in the rank, cannot be avoided. Nevertheless, in all this chaos there is strict harmony and consistency, although what is happening on the board is difficult to explain.
More here https://www.chess.com/blog/Pokshtya/battle-of-the-kings-is-a-chess-variant-that-drives-you-crazy
and here https://www.chessvariants.com/rules/battle-of-kings-