Transmuting kings = a king threatend by an opposing unit (i.e.,
a king in check) can only move in the same way as the threatening
unit. While not in check the king moves as normal kings.
(For example, a king currently threatened by a knight moves
only as a knight when escaping check, not as a king.)
Grasshopper = a piece that moves on queen lines by jumping over
the first unit encountered and arriving on the square immediately
behind this unit.
NightRider = a piece that can make a number of successive knight moves
as long as they are all in the same direction.