Facing the Tyranid invasion of Hive Fleet Jormungandr in the late 41st Millennium, they accepted the uneasy truce proposed by High Admiral Vortigern Hanroth to join his loose alliance of various fleets with their Shadowcruiser starships.
In 995.M41, during the subsequent Battle of Black Nebula, Hanroth and his unlikely allies shattered Hive Fleet Jormungandr.
Nevertheless, less than five Terran years later Hanroth himself would lead the obliteration of the Cimmeriac civilisation in the name of the Emperor.
- Codex: Tyranids (5th Edition), pg. 26