The Jagaroth were an ancient race of advanced, warlike, cyclopean humanoids.
In his fourth incarnation, the Doctor described the Jagaroth as "a vicious, callous, war-like race". (TV: City of Death) The Twelfth Doctor later commented that humans would have looked like "monstrous creatures" to the Jagaroth; (COMIC: The Swords of Kali) Scaroth deemed the Jagaroth "infinitely superior" to humanity. (TV: City of Death)
Jagaroth were lumpy humanoids with green, veined skin covered in small tendrils. They had a single eye on their forehead and a large flap on either side of their face. They had deep, echoing voices. (TV: City of Death) They were possibly also a time-sensitive race, able to see into the future. (AUDIO: Prisoners of Fate)
The Jagaroth had very advanced technology, capable of human disguises and holographic technology. They also had access to advanced knowledge and impressive intellect. Scaroth was able to fool the humans and advance humanity to the proper technology level using his wits and knowledge. Though they knew the concepts of time travel, they did not have the technology level. (TV: City of Death)
The Tenth Doctor remembered the Jagaroth as one of the "Old Ones", species which had "risen" to prominence in the Dark Times and later fallen from grace somewhat. (PROSE: The Knight, The Fool and The Dead)
At some point in the distant past, following the era of the Fledgling Empires, the Jagaroth had constructed a massive interstellar empire only for it to grow beyond their ability to control. When their subjects rose up against them, the Jagaroth were reduced to only one ship. (PROSE: The Whoniverse) This ship made it to the primordial Earth, having tracked a Racnoss energy signal, (PROSE: City of Death) four hundred million years prior to the 20th century, (TV: City of Death) due to the presence of the Empress of the Racnoss and her brood having hidden at the centre of the Earth earlier still in its history. (TV: The Runaway Bride)
Finding it unsuitable for habitation, they planned to leave the planet. However, their atmospheric thrust motors were damaged and inoperable, and they decided to take off using their warp-drive, against the advice of Scaroth, their warp-field operator. The attempt to do so deep failed, and the ship exploded. The massive burst of radiation generated by this explosion catalysed the development of life on Earth.
It also propelled Scaroth into the Time Vortex, where he splintered into twelve incomplete aspects of himself, each trapped at different points in the timeline of Earth. These aspects of Scaroth pushed forward the development of humanity, so that technology sufficiently advanced for time travel could be created and he could return to stop his past self from entering warp drive, and in turn save his people. He was eventually able to travel back in time to the moment before his ship's explosion, but was stopped by the Fourth Doctor, Romana II, and Duggan. After the latter knocked him unconscious, Scaroth was returned to 1979, where he was killed by his servant Hermann, in turn destroying the time field. This allowed the creation of life on Earth to go unaltered, and brought about the extinction of the Jagaroth. (TV: City of Death)