Hactrix was a transparent, red, tetrahedral crystal that could hover telekinetically. It spoke by altering the speed and oscillations of atoms in its crystal lattice. It was invited by the Eighth Doctor to the founding of the Institute of Time.
The Doctor travelled ahead to the end of the universe and found that Hactrix and the other members of the Institute had committed suicide. When he returned back to his original time, he refused to tell his friends what he had found. (PROSE: The End)