Jagaroth

The Jagaroth were an ancient race of advanced humanoids.

Biology
Jagaroth were lumpy humanoids with green, veined skin. They had a single eye on their forehead and two large flaps which became vestigial when they became sentient. They were on each side of their face. They were also covered in small tendrils. They had deep, echoing voices. (DW: City of Death)

Technology
The Jagaroth had very advanced technology, such as 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 just his wits and knowledge. Though they knew the concepts of time travel, they didn't have the technology level. (DW: City of Death)

History
At some point in the distant past, the Jagaroth were involved in a war amongst themselves which nearly wiped out the species. Despite this, some of the Jagaroth were able to make it to the primordial Earth, four hundred million years prior to the 20th century. 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 create a stable warp-field so deep inside the planet's gravity-well failed, and the ship exploded. The massive burst of radiation generated by this explosion catalysed the development of life on Earth and propelled Scaroth into the Time Vortex, where he splintered into twelve incomplete aspects of himself, which were trapped in different points in in the timeline. 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 save his people. (DW: City of Death)

Behind the scenes

 * Scaroth said the Jagaroth ship was destroyed four hundred million years before the 20th century, which apparently started life on Earth. In reality, multicellular life such as fish had long since developed on Earth. This timing error is similar to the problem of the age of the Silurians.