The Thals originally inhabited Skaro with the Kaleds.
Thals appear identical to Humans and Kaleds. Previous to the Thousand Year War the Thals tended towards blond hair. Afterwards, all Thals had blond hair. The Thals believed that, following the war, radiation had mutated them into perfect beings, though after a stage of advanced mutations.
The Thals had written records going back nearly half a million years. Oral history and later records would describe the Thals as originally warriors, while the Kaleds (known to the Thals as the "Dals") were more orientated towards philosophy and science.
Thousand Year WarEdit
The Kaleds and Thals engaged in a Thousand Year War. Though they originally used advanced technology, towards the end of the war they were forced to use technology that was more primitive. By the end of the conflict, both Thals and Kaleds sheltered in domed cities overlooking a wasteland.
The war concluded when Davros, a Kaled leader, gave the means to destroy the Kaled Dome to the Thals. They used this compound in the missile they sent to the Kaled city, and destroyed it. In "retaliation" for the destruction of the base, Davros ordered the Daleks to destroy the Thals. Led by a Thal, Bettan, Thals and Mutos joined together to fight the Daleks, who were trapped in the Kaled bunker after an explosion, defeated but still alive.
The Thals renounced violence and became pacifist farmers living in small bands on a less radiated plateau. They would not even engage in violence for self-preservation and showed no curiosity about the Dalek City or its inhabitants.
After a particular bad year for crops, the Thals were forced to move to the Petrified Forest near the Dalek City in order to look for food. Ian Chesterton, a companion of the Doctor, used moral blackmail to force the Thals, Alydon into particular, into putting together an expedition into the Dalek City in order to fight against the Daleks who intended genocide against the Thals. The Thals invaded the Dalek City, cut off the power, and so deprived the Daleks of the static electricity they needed to live.
By the 26th century, the Doctor's visit was considered a legend.
In 2540, the Doctor and Jo Grant met a taskforce of Thals on Spiridon where a small group, led by Taron, had landed to destroy a Dalek army massed on that planet. The Thals had, by then, reclaimed Skaro. They had only recently developed spaceflight, and the mission to Spiridon was the furthest they had ever gone; Earth was unknown to them, believed to be simply part of the Doctor legend.
The Thals were said to have relocated to another homeworld, sometime before the Renegade-Imperial Dalek civil war. In their absence, the Doctor had Skaro destroyed.
In reality, a decoy world named Antalin had been destroyed. The Thals were known to have survived past Antalin's destruction, and fought the Daleks on Terakis.
The real Skaro was destroyed in the Last Great Time War between the Daleks and the Time Lords.
It is unknown what became of the Thals after this point.
During the Thousand Year War, Thals were equallly as brutal as the Kaleds. They had no qualms about using the slave labour of Kaled prisoners and Mutos to build the neutronic missile, even when they knew the radiation from the missile would kill the slaves.
As noted above, after the war, the Thals developed into a peaceful people concerned with knowledge and living in small bands.
Centuries later, they revered the Doctor and his companions as heroes and referred to the prior event as the First Dalek War. Turning full circle, they had once more adopted a militaristic way of life, though only, it seems against the Daleks.