In Halcyon 6 deal with combat: Engage in tactical fleet or ground combat to resolve hostile events, create a foothold in the sector and ultimately defeat the Enemy Mothership!
The Mothership is a planet-sized biomechanical amalgam that acts as both a base of operations and a commanding force of the Chruul.
The Mothership was a sister starbase to Halcyon 6 created by the Forerunners that inhabits another dimension. The Chruul race found and seized this deserted base for themselves only to be mind controlled into doing it's bidding, effectively becoming the "hive" of this new "queen".
The starbase modified it's appearance by coating itself in a Chruul exoskeleton, and launched an assault on the Factions.