The Tanssor clan was an Ice Warrior clan from Mars.
Ixyldir was a warlord of the Tanssor clan. He led the clan from Mars to colonise a new planet. He tried to alter the technology that humans were using to terraform Hereafter to make the planet more like Mars. After being shown the error of his ways by the Eleventh Doctor, Ixyldir took the Doctor's advice and moved the clan to Atrox 881, which was already similar to Mars in climate.
Nine-thousand years later, Azylax was the leader of the Tanssor clan. He once met the Doctor and personally named him the Belot'ssar as a mark of respect, so that future generations would know him and tread carefully. (PROSE: The Silent Stars Go By)