In Troll Chess, all the pieces have this ability.
Each piece has a level, based on its relative rank in the Chess army. Pawns are 1, F or W are 2, N or B are 3, R is 4, Cardinal (Knight + Bishop) is 5, Q is 6, and K is 7; and so, when you make a move that would have been a capture, you subtract the attacker's current level from the defender.
If you have only demoted a piece, and not removed it from the board, both the attacking piece and the defending piece remain on the same square. If there is more than one enemy piece on a square, you can demote them all at once.
Because the Colorbound Clobberers have an unusual material balance, you may not choose them for this game.
When a piece is demoted to a rank that includes more than one type of piece, the capturer chooses which one.
After Black's move, all the White pieces regenerate, subject to the same conditions.
The regeneration rule is set up this way so that it is possible to capture things other than Pawns. When you demote a piece all the way down to a temporary Pawn, you will have one move to capture it; after one move it regenerates one level, and can at most be demoted to Pawnness.
When a piece is regenerated to a rank that includes more than one type of piece, the owner chooses which one.
If you have no legal move you simply pass, play no move at all; but naturally, if you have any move at all you must play it.
Although your level 2 and 5 pieces form part of your potential army, you start with no such pieces on the board. These two piece types will manifest themselves only temporarily, when some other type of piece is regenerating.
Choices for level 2 include Ferz, Wazir, Dabaaba, Wazir, Alfil, Crab, and Barc.
Choices for level 5 include any 4 picks from level 2, or the Bishop plus 2 picks from level 2.
Any piece composed of any two level 2 pieces is a level 3 piece; thus, WA, WD, Fibnif, and so on are level 3.
Likewise, any piece composed of 3 level 2 picks (or a B plus one pick) is a level 4 piece; and either 5 level 2 picks or a R plus 2 picks or a B plus 3 picks makes a level 6.
You can have unpromoted Pawns on the last rank -- pieces that were "captured" there. If your King has been turned into a Pawn, you may have the opportunity to heal it at once by moving it to the last rank and promoting it to a King (you must promote, and promotion to anything else loses).
Promoting anything else to a King is risky because if you lose either king you lose the game. In the endgame, however, it is often a very good choice.
However, my analysis indicates that the immediate attack with 1 e4 e5 2 Qh5 does not accomplish anything, although it does reveal an interesting tactical wrinkle: 1. e4 e5 2 Qh5 Nc6 3 Qxf7 Kxf7 4 f7xg8=Q with instant healing. Of course, 4...Kf7xg8 5 Bf1-c4 Rh8xg8 eliminates the Q, and 6 Bc4xg8 merely demotes K to R and R to level 2 (A,W,F,D, Crab, or Barc) and level 2 (the Ng8) to P.
A raid by a single unsupported piece can usually accomplish little, no matter how strong the piece is. The reason is that you can demote a piece all the way to P, but then you have only one chance to capture it; and you're on the same square as your target, so you can't do anything.
A "promotion raid" would be another tactical ploy; for example, 1 Nc3 Nf6 2 Nb5 Nc6 3 Nxa7 Rxa7 4 a7-a8=Q (and now, 4...Ra7xa2!? gets interesting). The idea is to get your piece demoted to P on the 7th rank, and then promote it to a higher rank than it started with.
Because the premature promotion raid with N could be foiled (for example, simply don't capture on a7 in the example), something more sophisticated is required. 1 e4 e5 2 Bc4 Nf6 3 d3 is an idea, but simply 3...Bc5 4 Bf7 Kf7 5 f8=Q Qxf8 followed by Rh8xf8 puts paid to the ugly troll.
I can't be sure that isn't true of the midgame, but in the endgame, things work out pretty well. King versus King is a draw, of course, but you might notice that if one K moves next to the other it loses at once in almost all cases. This means that K + P versus K is also a win in almost all cases.
K + B versus K, or K+N v K, are draws; K + R versus K looks like a win.
Because K+PvK is a win, we know that the game is winnable. All in all, it appears to be playable.
In addition, the difficulty of killing Trolls means that all the power remains on the board; Pawns will be difficult to keep; Knights can sacrifice themselves in order to promote themselves to Queens; and running the K down to the 7th rank in the midgame is a powerful defensive plan. In other words, the play of the game is not at all Chesslike.
I cannot close without saying "The bite-covered troll rises from the dead!"