Killer King
This secret achievement is tricky.
In order to get this one you need to deliver checkmate by discovery with the king in a live game (rated or non rated).
There are many combinations and ways to accomplish this, but it's also not so easy to get them (especially when people resign before you have a chance 
)
Here is an example of a game that went bad for black:
The white king can move to G1 or G2 and it will be checkmate (rook H1 discovery).
This would grant the player the Killer King achievement.
