The Diagonal mate - at first glance it might seem like just another variation of the Corridor mate but that's not the case.
The Diagonal mate is a checkmate where the attacking piece closes off a diagonal with the king (look at diagram 1). So initially it appears to be a Corridor mate. But there's a twist - the diagonal can be controlled by a Knight! Look at diagram 2 - here the Knight has taken control of a two-square diagonal with the King. This means a Knight can deliver a Diagonal mate! Quite unexpected.