> The grey knight would be immortal and could not be captured.
If grey pieces will make moves they should be mortal. In the DeadWall variant https://www.chess.com/clubs/forum/view/4pc-variant-deadwall we have unremovable pieces, but they stand still. Otherwise they may, for example, stalemate some player.
Dashes, is it possible (do-able) to add a neutral enemy piece? Sort of like a Grey Army, A.I. controlled piece that would attack any nearby human player pieces that step in range of an attacking square.
For example, imagine a grey knight that sits in the center of the board. It attacks any nearby player piece that either steps on or in range of an attack-able square. A knight can jump to one of eight squares. If a human player's piece is two knight jumps away, then the Grey knight hops to one of its attack squares and threatens the human piece (perhaps a customizable menu can allow a map creator/board creator to select which one of the eight squares can be the "designated attack-able square"). If the human player’s piece remains on the square that is being attacked, then the grey knight will capture it. After which, the grey knight will return to its post. If the human player's piece has entered one of the Grey knight's immediate eight squares, then the Grey knight captures the human player's piece.
The grey knight would be immortal and could not be captured. This feature would create scenarios, in custom games, in which 4 human chess players have to figure out how to work around the immortal Grey pieces (a Grey knight, a Grey rook etc.)
I thought about this after spectating several custom games in which human players navigated their kings with check counters through what appeared to be a maze. This suggestion could create interesting maze problems in which two players might have to work co-cooperatively in order to get their kings with check counters past a Grey knight guarding a particular passage.
And at the end of the maze?
In the final room, the Grey queen awaits weary maze runners …
The Complexity of Turns
It would always be the Grey Army’s turn to act (this is important because several human players could be located in different parts of the maze. And the Grey pieces need to be able to simultaneously attack several human player’s pieces as soon as they are in range of attack-able squares), but after a Grey piece leaves its post to deliver an attack, then that particular Grey piece would experience some sort of cool down, eventually returning to its post. In this manner, players would need to calculate in order to figure out ways of sneaking past either stationary or patrolling Grey army pieces.