Tachon was a Time Lord.
He was part of Xenaria's expedition to break the time loop surrounding Planet 5 about 12 million BC, so the Fendahl could be released and used against the enemy. Like all members of the expedition, he had been changed into a Elder Thing as camouflage during the time period.
He was killed when the dimensional engines for the birthing cradles they had brought for the War TARDISes fused. They returned to being spherical chambers a dozen metres, with everything inside, including Tachon, being crushed. (PROSE: The Taking of Planet 5)