The mothership is the Tal'darim faction choice for the capital ship class in the Legacy of the Void campaign, and the third choice with the carrier and tempest. As in the multiplayer, only one mothership can be active at a time.
The mothership is a terrifically powerful offensive vessel: it can attack while moving and its base attack does very high damage, enough to kill many enemies in one shot. Its Black Hole ability functions differently from the normal mothership, keeping units on the map in a stunned state so they may still be attacked, and its Thermal Lance does 200 damage to all enemy ground units in a line leading away from the mothership. It also has access to Blink, which makes it easier to quickly engage enemies or retreat from battle.
The mothership is a very powerful weapon, but should be used in tandem with other units, as it is easily overwhelmed with superior numbers, especially air units which its Thermal Lance cannot damage. It also costs a significant investment of resources to deploy, but is substantially cheaper than a fleet of carriers or tempests.
Alarak can summon the Death Fleet as a calldown ability, summoning a mothership and an escort of destroyers with timed life. The mothership has the same health stats as the campaign version, but with improved damage via upgrades and different abilities.