From Archon Arcana - The KeyForge Wiki

A card’s rarity symbol can be found at the bottom of the card, near the collector number. A card’s rarity (common, uncommon, rare, or special)is used by the deck-generation algorithm to determine how frequently it will appear in decks. Special cards have a different type of distribution and do not obey the game’s standard rarity rules.

KeyForge Master Rulebook v17.3 March 2024


(More information regarding card distribution can be found in Card Rarity)

There are three types of special rarity that the deck building algorithm uses, according to the Master Vault:

  • As of the release of Age of Ascension, Variant cards are assigned to a deck and then modified based on other contents of the deck. For example, the algorithm decides to assign an Ambassador to a deck, and then based on the non-Sanctum houses in the deck, an appropriate Ambassador is assigned. In Call of the Archons, the Masters were the only variant, and they were inserted slightly differently; Master of 1 was inserted into the deck, and then based on the contents of the deck, it was either left as Master of 1 or changed to Master of 2 or Master of 3.
  • As of the release of Mass Mutation, the vault returned a rarity actually called "Special". The only card with this rarity so far is Dark Æmber Vault.