1. killer queen. Deliver checkmate with a queen in a normal game of chess. A discovered checkmate counts, but not a checkmate delivered with a newly promoted queen, such as one in which the checkmate move is h8=Q# (that would be killer pawn). As a general principal for ALL killer pieces, (with the POSSIBLE exception of situations involving castling for a mate (see castle victory in the other checkmates section)) If you move a piece, and then you checkmate your opponent on the same turn, then you get the killer piece achievement for the piece that you moved (NOT the piece it became, if any) (and ONLY the piece that you moved, castling being the foggy area, but I'm pretty sure that's killer king AND castle victory, but there's a chance that it's only castle victory), regardless of any change in the piece's identity throughout the move.
Awards: Achievements: the killer pieces
2. killer rook. Deliver checkmate with a rook in a normal game of chess. A discovered checkmate counts, but not a checkmate delivered with a newly promoted rook, such as one in which the checkmate move is h8=R# (that would be killer pawn). Castling is another slightly foggy area, but you can see the entry for killer queen and castle victory (in the section other checkmates) for more on that.
3. killer bishop. Deliver checkmate with a bishop in a normal game of chess. A discovered checkmate counts, but not a checkmate delivered with a newly promoted bishop, such as one in which the checkmate move is h8=B# (that would be killer pawn). The entry on killer queen gives more background info.
4. killer knight. Deliver checkmate with a knight in a normal game of chess. A discovered checkmate counts, but not a checkmate delivered with a newly promoted knight, such as one in which the checkmate move is h8=N# (that would be killer pawn). The entry on killer queen gives more background info. Please note: Resigning is NOT checkmate!
5. killer pawn. Deliver checkmate with a pawn in a normal game of chess. A discovered checkmate counts, and also checkmates delivered by a newly promoted piece, such as one in which the checkmate move is a8=Q#. The entry on killer queen has plenty of other information, and the other killer piece entries (not including killer king) include more examples of getting killer pawn via promotion.
In this section, we will look at the killer piece achievements, which include 5 achievements and 1 hidden achievement. Please note that these achievements are NOT listed in the order they appear in your achievements section. The format for a section is always like so: FIRST I list all normal achievements in which ever order I please (however, stacking achievements are listed in order, more or less). (but you might find some other patterns) The first one or two will probably also have some background info at the end. Then I list all hidden achievements in whichever order I choose. (stacking order is still a thing) I might put other info elsewhere. The format of each individual achievement in a section will likely be similar. Hope this section helps you become a better chess player while getting you achievements!