The Dahensa, pronounced "DarHen-Sah," were a scorpion-like alien race with yellow glowing eyes and four arms, two of which ended in pincers while the other two ended in humanoid fingers. They had two sexes: male and female.
They created a probe device for the purposes of gathering information which they called the ranger. They launched one into Earth's atmosphere in 1903 in preparation for their invasion. It was seriously damaged either in transit or when it fell to Earth, specifically in Zarechny, Siberia, Russia. Due to the emission of chronon particles by its damaged power source, it began to detect and record humanity's future at a rate of approximately 1,000 years per day. The First Doctor told Ian Chesterton that once it reached the limit of its information retention capabilities, it would vent its information around the Earth and every human would gain precise knowledge of the future.
When Grigori Rasputin made contact with the device, he gained this knowledge, though the Doctor subsequently removed it from his mind by connecting the ranger to the telepathic circuits of his TARDIS. The Dahensa attempted to invade the Earth at least once more prior to the 30th century. (AUDIO: The Wanderer)
The Third Doctor and Sarah Jane Smith thwarted an invasion of Dahensa in London by setting up a sonic manifold feedback loop over the city from the Post Office Tower. (COMIC: Invasion of the Scorpion Men)