Here's the promotional blurb for the new set, Celestial Battlefield:
"Long have the Mid Realms suffered the effects of the titanic struggle between Guardians, Vierkun, and their armies...they have become shattered lands, where armies find it difficult to face each other, leaving conflicts unresolved. The Vierkun found a way to shift battles to the Celestial Planes, but when both opposing Vierkun attempt to shift the battle, the destination becomes uncertain, and the conditions for battle become unknown. In addition, Vierkun must often work together to defeat Avatars, powerful denizens of the Planes. This is the new stage upon which the Vierkun and their armies find themselves, and the results are often unpredictible, ensuring that only the strongest tacticians will survive..."
Celestial Battlefield is a Guardians expansion of 100 cards, including 2 new card types, Planes and Avatars, and other cards that make use of these new card types. This new expansion is fully compatible with the Revised Guardians set and all other Guardians expansions.