You may only have one knight in play at a time: if you have more than one knight at any time, you must remove all your knights except for one from the board. On your turn, instead of moving, you may permanently convert a bishop or rook you control into a knight.

At the beginning of your turn, you may choose to hide your knight. Knights may be hidden for up to three turns in a row. While your knight is hidden, its position is not shown on the board but is secretly written down each turn by you. Hidden knights may still be captured, but if a piece passes through a hidden knight, you may choose to reveal the knight’s location and automatically capture the piece that moved through it.

First appearance: 3rd Design Competition (nonfinalist, added to official rules by committee decision).
Creator: Alex Nisnevich
Tags: Aggressive power  Complex rules  Defensive power