I have a neat idea - "Shadow-Realm" - could use some help with ideas on how to flesh it out

Imagine a chess game where there is a "shadow-realm" that exists concurrently. But when pieces enter this realm, they leave the board and you have to keep track of where they are mentally. But there are only certain squares by which a piece enters/exists that realm. And certain pieces that are "shadow-pieces", with certain attributes unique only to them, begin play already in that realm, with certain requirements that must be met to "bring those pieces" into the real board. One such piece, the "assassin" can kill the opponent's king - effectively ending the game immediately  - merely by checking him. Therefore, for instance, it would behoove you to force your opponent's king into that realm (if at all possible) even before the assassin is brought from the shadow-realm into the real board, so that your assassin can snipe him.

 Thanks. The only problem is it's very hard to figure out how to actually design such a game.