From the campaign:
Spell Chess is an exciting addition to your Chess playing experience. It consists of a deck of cards that modify and change the normal rules of chess as you play. It creates exciting new challenges to the existing game, as well as opens up new emergent strategies and tactics.
Spell chess is a two player game consisting of your chess set and the Spell Chess expansion. The pieces are still the same but, by drawing cards from the spell deck you will change the game with an ever twisting chain of spells.