Passenger form

A Passenger was a living container with endless space for prisoners to occupy. Swarm and Azure used them to hold hostages during the Siege of Atropos. (TV: Once, Upon Time) After Swarm freed himself from imprisonment, and re-united with Azure, one remaining Passenger accompanied them to the Temple of Atropos. (TV: War of the Sontarans)

Siege of Atropos
to be added

During the Great Disruption
When Passenger joined Swarm and Azure in the Temple of Atropos, Swarm noted with satisfaction that this meant they numbered three. He seemed to act as a sort of henchman to the siblings, with Swarm using him as a threat against Inston-Vee Vinder, telling him to obey Swarm "unless [he] want[ed] to upset Passenger here". (TV: War of the Sontarans)

Appearance
Passenger was taller and broader than both Swarm and Azure, and had no crystalline growths on his skull-like face, which was dark gray. Instead a row of small gems were arranged vertically where his mouth should have been. As a result, Passengers did not speak. (TV: War of the Sontarans)