I recently purchased D. B. Pritchard's volume on Chess variants.  Dark Chess is quite popular on Brainking so there is the possibility that this variant created by Ralph Betza in 1978 might also be popular.  

Knights are replaced by invisible ghost riders.  Opponents are told when a ghost rider moves but the destination is not given unless a capture is made.  Pieces may pass over squares occupied by a ghost rider, even when castling.  (I assume this means queen-side castling since otherwise two pieces would be on the same square.)  Ghost Riders are subject to capture like all other pieces.  A player may, not mandatory since Ghost Riders ordinarily don't stop movement, reveal a ghost rider's position in order to block an enemy movement.  If a ghost rider checks the enemy king the check is announced but the location of the ghost is not given, unless the move also captured a piece.  A king in check from a ghost rider has the option to attempt capture of the ghost rider but loses if the wrong square is chosen.  

The book is unclear about what happens when a piece is blocked by a ghost rider.  There are several possible options.

1.  Choose a different move 

2.  Forced capture of the ghost rider by the blocked piece (I personally like this option which introduces some real uncertainty in movement.)

3.  Stopped just before the ghost rider.

Talen314