High Gallifreyan

High Gallifreyan was the ancient language of the Time Lords. It was not known by many; by the Doctor's era, it was virtually extinct, superseded by Gallifreyan.

The Doctor claimed High Gallifreyan once possessed immense power when correctly harnessed, such as raising Empires or destroying Gods, the truth of this is unknown.

The written form of High Gallifreyan resembles, to human eyes, a mixture of Greek letters and mathematical symbols. (DW: The Five Doctors)

Inside the Tomb of Rassilon in the Death Zone on Gallifrey, an old rhyme was written in "Old High Gallifreyan" which explained the outcome of the "Game of Rassilon". (DW: The Five Doctors)

Within the Doctor's rooms on Gallifrey there was a painting of a woman, holding a scroll with the words "Death is but a door" written in High Gallifreyan. (PDA: The Infinity Doctors)

The First Doctor wrote his Five Hundred Year Diary in High Gallifreyan to make sure no one could read it. (DW: The Power of the Daleks)

River Song could write in Old High Gallifreyan, which she used to get the Doctor's attention. Who taught it to her so far remains unknown. (DW: The Time of Angels)