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. (PROSE: The Taking of Planet 5)

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)