Scaroth, also known as Count Carlos Scarlioni and Captain Tancredi, was the last of the Jagaroth and planned to also become the saviour of his species. He was also the pilot of the Jagaroth ship that landed on Earth in prehistoric times and concurred to the rise of life on the planet; moreover, he claimed to have contributed to the technological development of the human race.
Roughly 400 million years BC, a Jagaroth ship piloted by Scaroth landed on Earth. Despite his ship's advice, he launched at Warp 3; the ship exploded, and in its control cabin, Scaroth was splintered in the time vortex.
Twelve fragments of him were scattered across time, though they could communicate through telepathy. The explosion provided the radiation that the primordial soup needed for the human race to exist. Throughout time, Scaroth's twelve selves pushed the development of the human race forward so that his twelfth self would have the technology to go back in time to the moment of take-off, preventing his splintering in time as well as the death of his race.
All of these incarnations, except for the Egyptian self, who appeared as a god on one occasion, disguised themselves as humans using a mask and skin into which he was able to fit himself. (TV: City of Death) His Egyptian self was manipulated by the Osirans to serve as a foreman in arranging the construction of the pyramids. (PROSE: The Sands of Time)
In Florence, Italy in 1505, Scaroth appeared as a human named Captain Tancredi. He convinced Leonardo da Vinci, a human painter, to paint six copies of his famous painting, the Mona Lisa. He stored them away for a later self to find. (TV: City of Death)
In 1799, Cardinal Catilin showed Marielle Duquesne a set of Egyptian manuscripts, found by Cardinal Scarlath and kept in the Collection of Necessary Secrets, that described "a world built by one-eyed supernatural horrors". (PROSE: Christmas on a Rational Planet)
His twelfth self was Count Carlos Scarlioni, a well-known figure in the art world in 1979. Thanks to him, genuine art works that had been missing for centuries started showing up. He was married to Countess Scarlioni, who never knew who he really was, thanks to little gifts every now and then and a little "excitement". He also gave her a micromeson scanner and asked her to wear it always.
Thanks to his other selves, he used the artwork to fund his experiments with time, run by Professor Fyodor Nikolai Kerensky. Having found the other six Mona Lisas which were bricked up in his cellar, he planned to steal the one in the Louvre.
He would then sell them to the seven people in the world who would pay anything for the Mona Lisa, and since the one from the Louvre would be stolen, each would think he had the original, and Scaroth would make a hundred million dollars off each panting which would further fund his time experiments to a point where he could go back in time and prevent his past self's mistake.
He met the Fourth Doctor and Romana II when they tried to find the crack in time that his experiments had caused. Romana was captured and made to build a field interphase stabiliser to make the artificial time field Scaroth had created stable, allowing him to travel back in time, as he threatened to destroy Paris if she didn't comply.
Having been convinced by the Doctor, and having seen pictures of Scaroth's true form, the Countess turned against Scaroth. He used her bracelet to kill her. Scaroth travelled back in time to try to stop the explosion that splintered him across time; although Romana had sabotaged the field so that it would only take him into the past for two minutes, that would be enough time for him to avert his past mistake.
The Doctor got there first with Romana and Detective Duggan. After the Doctor failed to convince Scaroth to stand down on his own, Duggan knocked Scaroth unconscious. His time in the past then ended and he faded back to 1979. When his servant Hermann saw his true form, he attacked Scaroth with a grenade, and Scaroth was destroyed along with his time field. With Scaroth's death, and history left unaltered, the Jagaroth became prematurely extinct (TV: City of Death)
The image of Scaroth was among the wealth of memories which the Thirteenth Doctor, while trapped by the Spy Master, used to overwhelm and break free from the Matrix in the ruins of the Capitol on Gallifrey. (TV: The Timeless Children)
Scaroth was an extremely callous being who didn't seem to care for anyone but himself; when the Doctor asked him if he knew that putting his entire being back together and preventing the destruction of his ship would cause all life on Earth to have never existed, Scaroth's response was that he did know - he just didn't care. He was arrogant, sarcastic, manipulative and extremely sadistic. He despised humanity, believing them to be primitive and unintelligent, and even murdered his own henchmen on some occasions.
In keeping with his sadistic personality, Scaroth also had a remarkably cruel sense of humour, noting that the only reason he spared Duggan's life was because it was "unfortunately not possible to kill him twice" and joking about the Countess' death shortly after killing her. (TV: City of Death)