Passenger form

A Passenger form, often shortened to "Passenger", was a living container with endless space for prisoners to occupy. It was a bioform, (TV: The Vanquishers) which was long forbidden by the end of the Dark Times, (TV: Once, Upon Time) yet still remained a concern for the Grand Serpent's fleet (TV: The Vanquishers) in the 21st century. (TV: Survivors of the Flux)

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)

In the 21st century, Inston-Vee Vinder was trained on how to escape a Passenger at the Academy, (TV: The Vanquishers) and Bel encountered them on her last deployment. (TV: Village of the Angels)

Siege of Atropos
During the Siege of Atropos, Swarm and Azure used five Passenger forms to hold hostages. Each of them held millions of people captive inside them. Despite this, Swarm destroyed two of them without a second thought. However, the Fugitive Doctor was able to break the siege by concealing the Mouri inside one of the Passengers, allowing them to retake their positions and thwart Swarm and Azure's plans. (TV: Once, Upon Time)

During the Great Disruption
By the time of the Flux, at least one Passenger remained in the service of the Ravagers. This Passenger (named simply "Passenger") joined Swarm and Azure in the Temple of Atropos, Swarm noted with satisfaction that this meant they numbered three. Swarm used them as a threat against Inston-Vee Vinder, telling him to obey Swarm "unless [he] want[ed] to upset Passenger here". It was also used to hold Diane prisoner, a fact Swarm taunted Dan Lewis with. (TV: War of the Sontarans) The Passenger later departed along with its masters. (TV: Once, Upon Time)

Trapping survivors
After arriving on the planet Puzano, Azure used the promise of safety to lure many of the Flux's survivors into the Passenger's transportation field, taking them prisoner. Bel recognised the Passenger for what it was and was able to prevent Namaca from being taken in. (TV: Village of the Angels)

Absorbing the Flux
The Thirteenth Doctor used a Passenger to absorb the remainder of the Flux, due to the Passenger containing endless matter that combatted the antimatter of the Flux. (TV: The Vanquishers)

Appearance
Passengers were taller and broader than both Swarm and Azure. They somewhat resembled Ravagers, with ornate clothing and a skull-like face, which was dark gray. However, the face did not have crystalline growths like Swarm and Azure's; instead, a row of small gems were arranged vertically where their mouth might have been. As a result, Passengers did not speak. (TV: War of the Sontarans)