Object:
Western Chess, except that Pawns may capture their own teammates, generally promoting into a higher skilled piece, and that Kings and Queens are each prone to lone checkmate or else capture(s).
Both sides start with two Kings. All Kings (and Queens) are freely capturable until only one (of a type) is on-board, leaving a either lone King or Queen co-checkmate possibility.

A Pawn-captured friendly Knight changes into a Bishop, or King.
A Pawn-captured friendly Bishop changes into a Rook, or King.
A Pawn-captured friendly Rook changes only into a King.
A Pawn-captured friendly King changes into a (still-royal) Queen [prone to checkmate], or a (non royal) Knight, Bishop, or Rook.
A Pawn-captured friendly Queen turns into a (non-royal) Knight, Bishop, or Rook.

Owing to the possibilities of multiple Kings and (royal) Queens, checkmate and check were restructured.
Win by either checkmating a lone King OR a lone Queen. Multiple Kings or Queens are merely captured.
Kings and Queens have separate 'thrones'.
For example, three active kings and one queen - any two kings may be captured (the last remainder checkmated), the queen is separately attackable by checkmate. (And vice-versa.)
A potential lone King or Queen may not be left under check through changing a second King or Queen.
However, a second active King or Queen may be generated while a lone royal is under check.

Castling works with both starting Kings - keeping each King's farther positional Rook difference. Each side must maintain at least one King on-board at all times. If a Queen is added, then one Queen is required from then onwards.
8th-row Pawn promotions including (royal) Queen, and 'En Passant', still exist.

Subvariant: Failing Hands Chess - with Reserves (4 Pawns, 2 Knights).
Four extra Pawns and two Knights are available, in reserve. Each may be added to their empty starting areas as wanted. Reserve pieces may not be dropped where an enemy piece is currently sitting in an attack-prone position.

Subvariant: Failing Hands Chess - Dunsany's Battle, with 6 Reserve Pawns.
White's side is replaced with 32 starting Pawns - plus another 6 reserve Pawns (for the second row, no immediately capturable enemies). Black moves first.
White Pawns, upon their second row, do not have initial two-step choices.
White's pawn promotions, etc., have the same 'Failing Hands Chess' alterations. One Queen or King is prone to checkmate, multiple Queens or Kings may be captured until one is left.
Black wins by eliminating the side, or checkmating a (royal) lone Queen or King.