The Dellacoi were a species of symbiotes who could shape reality to their whims.
In their natural form, Dellacoi were grey amorphous creatures, something like a slug. They were able to enter a host through the mouth and mentally connect to them. With a host, the Dellacoi was able to survive in hostile environments, while the host would be able to survive fatal damage.
The Dellacoi were used to warm climates and required host organisms to traverse hostile environments. Leet described Cardiff as "the cold wastes of this planet".
The Dellacoi were also able to create objects out of pure thought. Though these creations had the appearance and abilities of real objects, they had no actual substance. They could also use the thoughts of their host, but they would pass into the control of the host, meaning the Dellacoi would be unable to uncreate them. (PROSE: Bay of the Dead)
The Dellacoi travelled in life-shells, football-sized orbs which allowed them to survive without a host. The life-shells were capable of travelling through time rifts, regenerating and telepathically communicating with the Dellacoi. (PROSE: Bay of the Dead)
Circa 2009, the Dellacoi Leet was caught in the Cardiff Rift and crashed to Earth, landing on the Regal Cinema during a zombie movie marathon, killing 63 people. In order to survive, Leet entered Oscar Phillips, saving his life but keeping him in a coma until he was able to heal. In the mean time, Torchwood Three took his life-shell back to their base.
3 months later, the life-shell had repaired itself enough to send a message to Leet. Leet used the most recent memories of Oscar to create search units to get the shell back. Unfortunately, the most recent memories of Oscar were of zombies, which were created but immediately went on a rampage. In order to contain the damage, Leet created a time energy barrier, stopping anyone from leaving or entering Cardiff. Eventually, Ianto Jones and Jack Harkness were able to draw the connection between the regenerating life-shell, the hunting zombies and the survival of Oscar. They were able to bring the life-shell to St Helen's Hospital where Oscar was kept, but Leet was unable to stop the zombies himself, forcing Oscar to kill himself to destroy them. (PROSE: Bay of the Dead)