In Rex Multiplex, a pawn can also promote to king, so more than one king of the same color may be present on the board. A move, giving check to different kings, is allowed only if all checks can be parried on the next move. Mate can only be given if all kings are mated in the same move. In parrying a check or a mate for a part of the attacked kings, it is not allowed to expose any other king of own color to any (additional) check or mate. In case there is no other move of defense available, either the position is mate or the checking move was illegal.
Illegal Cluster (Rex Multiplex)