Umbra was the second set for the Rage CCG. It was the smallest of the sets at under 100 cards and was only sold in boosters. Every Garou tribe got one new character in Umbra, plus two new Fera were introduced with a single character each (the Corax and the Nuwisha).
Despite its small size, Umbra introduced three new card types and expanded one subtype.
Caerns first appeared in Umbra and allowed Garou to step sideways and enter the Umbra. The Umbra had been introduced in the Limited edition, but only as a sort of 'safety zone' that you could flee to via the card Umbral Escape or Umbral Quest. Other that, you could not do anything else there. Umbra fleshed it out into its own parallel world that you could now visit and explore. Caerns have appeared in every set since Umbra.