The Intruder Zapper.
It sits transparently on a1 watching the board 24/7 with a radar device. Whenever a non-standard chess piece appears on the horizon it zaps the intruder and returns to "silent" mode. It requires no instructions or operation - but when a red light turns on, replace its batteries.
Warning! Turn off in the vicinity of experimental chess pieces.
The troll: it cannot be taken but can't capture. Moves like a rook (for ??? Effect searrch my profile)