The Vortex Dwellers were advanced beings who lived in the Time Vortex.
Vortex Dwellers had an ooze-like form that constantly changed in colour. They could change their shape to a degree, an ability which one Vortex Dweller used to become more humanoid. Their normal method of communication was with electrical shocks. Due to high voltage (at least fifty thousand volts), they were generally unable to communicate with lesser beings. They also produced chronon energy.
Vortex Dwellers were capable of advanced control of space and time. They weren't technically capable of changing time, but they could re-arrange it. Vortex Dwellers could bring people who had died back to life, though they would still have scars and memories of the deaths. They were also capable of closing time rifts. (PROSE: The Undertaker's Gift)
In 2009 a young Vortex Dweller came to Cardiff via the Cardiff Rift. It accidentally killed two people before Torchwood brought it to the Hub. As it was unable to communicate with the humans, it simply sat in its cell and waited. In the meantime, its parent started looking for it, but could not track it as it was in the normal universe.
When the Undertaker's Gift was activated, it tore open a hole in the Rift. The parent Vortex Dweller detected the event and investigated. Jack Harkness reunited the Vortex Dwellers. He tried to communicate with them, dying repeatedly but finally getting their help. The Vortex Dweller repaired the damage from the Undertaker's Gift, brought back several people who were killed and stopped the Hokrala Corp from sending anything else into the 21st century. It also offered to seal the Rift, but Jack declined. (PROSE: The Undertaker's Gift)