Transmuting kings = a king threatend by an opposing unit (i.e.,
a king in check) kan only move in the same way as the threatening
unit.
EquiHopper = a piece that moves by jumping over another unit
and arrives on the square the same distance behind this unit.
The path must be free both from the equihopper to the unit jumped
over and from this unit to the arrival square. (For example, in the
diagram position, the equihopper can move to f2 by jumping over the
black king, and to d8 by jumping over the white king. If the white
king was on g6, the equihopper could jump to f8.)
NightRider = a piece that can make a number of successive knight moves
as long as they are all in the same direction.