This is an enhanced version of a previous explanation:
https://www.chess.com/clubs/forum/view/the-time-thief-explained
The Time Thief moves like a Queen.
The Time Thief can be captured by other pieces. But the Time Thief itself cannot capture other pieces in a normal way.
However, the Time Thief has a special way of postponed capture:If an enemy piece is in reach of the Time Thief, and does a move, the Time Thief can as response revert the entire move and then capture the piece.
In this diagram, the black Rook is in reach of the white Time Thief. Suppose Black plays Rf2.
As response, the white Time Thief can revert Black's previous move. So it would move the black Rook back from f2 to f7. And then it can capture the black Rook on f7. As a result this would be the position:
The Time Thief must do this capture on the very next turn, or the right to do so is lost!
The Time Thief cannot revert a move after being captured itself:
After Rxh3, the Time Thief is off the board, and has no powers to revert that move and capture the Rook.
If the Time Thief reverts a move, it reverts all aspects and everything what happened with that move. It really turns back time.
In this diagram, Black might play Rxi7. The Time Thief can revert that move and capture the Rook on f7, but that also means that the Queen comes back on the board on i7 : the Queen is restored by the Time Thief.After Rxi7 TTxf7:
In the following diagram, Black has just played pawn f7-f5:
Before f7-f5 was played, the Rook was in reach of the Time Thief. but the Rook did not make any move. So the Time Thief cannot capture the Rook.The pawn did move, but did not come from a square in reach of the Time Thief. So the Time Thief can also not capture the pawn.
In the following diagram, the White King is in check.
White might consider to play ixj7, thinking that his King is indirectly protected by the restoring powers of the allied Time Thief. This is not allowed. A Time Thief cannot restore his own allied King. In this position, White must get out of check.
A King in reach of an enemy Time Thief, is not yet in check.
White to move here. With any move by the White King, White would put himself in check, because it allows that the black Time Thief will capture the white King. This position would be Stalemate, as White has clearly no legal moves left.
Note that in the same position with black to move, Ke4+ would be an illegal move for Black. Although the White King seems to be frozen by the Black Time Thief, he still gives check, if the Black King would approach to an adjacent square.
Fairy piece interactions
Time Thief and Spirit
Scenario: a possessed piece is in reach of a Time Thief and the Spirit leaves that piece.
Additional rule 1: If the Time Thief is unpossessed then the lone Spirit is invulnerable for the Time Thief. The Time Thief can't take.
Additional rule 2: If the Time Thief is possessed then the Time Thief can capture the Spirit. The piece that was possessed by the Spirit will as a consequence also be captured. This is a circumstantial double capture and is allowed. If the possessed piece was a Dwarf then the circumstantial double capture is still allowed whether or not the possessed Time Thief had an allied co-attacker.
Other scenario's with Spirit and Time Thief are already covered by the existent rules.