The Changeling starts out as a Thorny Rose, which moves like a king, but can only capture like a bishop. Upon moving, it turns into a Rook. When it moves again, it turns into a bishop. When it moves yet again, it turns into Queen. On moving a fourth time, it turns back into a Thorny Rose, and so on.
The Drop Rule: When a player's Changeling is captured, that player may, on any subsequent turn drop it back onto any empty square (this counts as a move). However, the Changeling can only be dropped in the same form in which it was captured. Bishops can only be dropped as bishops, rooks can only be dropped as rooks, and so on.
The object is checkmate the opponent's Archbishop. Whoever does that first wins.