Huldrans were a telepathic species. This was not their true name, but what Nimrod chose to call them based on Professor Harket's research.
They were small, thin and blue. When they spoke, it sounded like they were singing. They could produce a slime through their skins which could incapacitate or kill, depending on how much came in contact with a non-Huldran.
Their spacecraft travelled by creating a portal that allowed them to go through a dimensional vortex.
A Huldran ship crashed in Norway and was salvaged by the Forge, which also killed one of the survivors while experimenting on it. More Huldran came to avenge its death, but were captured by the Forge as well. The Seventh Doctor helped them to escape, warning them to stay away from the vicinity of Earth in the future. (PROSE: Project: Lazarus)