The Nightrider is a very popular fairy piece which rides
along straight paths made of Knight steps. Invented by T. R. Dawson
A Nightrider on a1 may go to b3, c5, d7 (along a 1st Knight direction)
and to c2, e3, g4 (along a 2nd Knight direction). As all riders, it
can only go to c5 if b3 is free, and to d7 if b3 and c5 are free. Thus
a Nightrider is to a Knight what a Queen is to a King.
It is depicted as an upside-down Knight. Usually it is denoted by
N when Knights are denoted by S. To
illustrate this, consider the following position:
(White Nightriders b5, c1, g2)
Here the bl. King is under triple (!!) check by Nc1, Rf3 and Pe6.
It is not under check by Nb5 because the bPd6 blocks this Nightrider's
path (and then the bPd6 is pinned...) Bl. cannot answer Kxe6 because
the Ng2 protects e6.
The triple check can only explained by last move -1. f5xe6
ep, preceded by -1 ... e7-e5 answering -2. N~-c1+, or maybe
the discovery check -2. Re3-f3+.
In retros with Nightriders, the Ns are sometimes used to replace Knights
in the initial position (like e.g. Maos). In other
cases, and unless the stipulation states otherwise, they are assumed
to result from promotions.