From Lexicanum

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Excommunicate Traitoris is the term used in reference to Space Marine Legions or Chapters that have betrayed the Imperium through heresy, mutation or any other act the Imperium deems unforgivable. Several forces have been deemed Excommunicate, such as the Traitor Legions¹, the Flame Falcons, the Relictors and the Soul Drinkers². They are also known as Renegade chapters.

Sources

• ¹ Codex: Space Marines (3rd Edition)
• ² Soul Drinkers Series by Ben Counter