The Therovian Quest
Ixon was an ice planet which was the homeworld of a hostile, green-skinned humanoid race led by Ixa. It was also home to a type of moss which could be made into the cure for a plague afflicting the Therovians. It grew at the centre of the planet, which was also home to furry creatures with large tusks.
Although many travellers had gone to Ixon, few had returned, as Ixa’s men would capture any strangers they found. Grig, the First Doctor, John and Gillian came to Ixon to collect the moss to cure the Therovian plague, escaped Ixa’s clutches and returned to Theros. (The Therovian Quest)