Not really specifically a new piece, but a new form of promotion where any major/minor piece can promote upon capturing another piece of equal or greater value, except for pawns which require normal promotion rules.
If knight captures knight/bishop/rook/queen, he becomes a paladin which still moves like a knight but upon defending another piece, if that other piece is captured, he can choose to be captured instead while keeping the attacked piece safe and capturing the attacking piece.
If bishop captures knight/bishop/rook queen, he becomes pope and can move like the pope I mentioned above.
If rook captures rook/queen, it becomes Castle (or Palace if that name is confusing) which moves just like a rook normally does but can also capture diagonally only in melee range.
If queen captures queen, she becomes Empress which still moves like a queen, but while alive, your king can't be checkmated, and if your king does get captured, the empress takes on the checkmate conditions.
If king captures queen, he becomes Emperor which allows him to move two squares in any direction.
Since most situations where you capture a piece of equal/greater value were a trade, most situations you wouldn't manage to keep a promoted piece alive. But it would cause hesitance for certain sacrifices. For example early game if middle pawns are cleared up and queens are in starting positions, you might not want to trade queens or else you'd be promoting their king to emperor.
The pope - upgraded bishop which is still limited to his starting color, but can go to any of that colored square on the entire board except if the path is blocked by another piece (there must be some diagonal path that leads to the destination but can be ANY path doesn't need to be a fully straight line can zig zag paths in a lightning bolt shape for example). At the start of the game white chooses which of his 2 bishops he wants to be the pope followed by black choosing his.
A pawn can promote to pope but ONLY if your current pope is killed. Otherwise if your other bishop is still alive he'll become pope upon death of your first one.