The Life Force were a faction of dimension explorers from the Eight Thousand and Ninety First Universe. They were biologically so far from the Doctor's universe that the Second Doctor found their bodies hard to describe, being "writhing vortex of something slimy and sticky" which thrived in a methane and chlorine atmosphere. Because oxygen was poison to them, they wore multi-limbed spacesuits to survive in the Doctor's universe.
They mapped a thousand tangential doors connecting the Space Time Universes of the multi-dimensional Cosmos, with the thousand and first door leading to the Doctor's universe. (PROSE: A Thousand & One Doors)
A member of the Life Force was among the aliens stuck on Denossus when it was transported to the Time Vortex by the Vortexians. They would remain trapped for over twenty years until the Vortexians were expelled by the Doctor. (COMIC: Land of the Blind)