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 the Old Swarm freed himself from imprisonment, renewed himself into a new body 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. Swarm used 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
Passengers were taller and broader than both Swarm and Azure. They somewhat resembled Ravager, with ornate clothing and a skull-like face, which was which was dark gray. However, the face had no crystalline growths like Swarm and Azure's; 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)