Paarthurnax is a dragon also called "the Old One" by other dragons, is the leader of the Greybeards. He served as Alduin's lieutenant during the Dragon War, during which he was said to have committed and witnessed unspeakable atrocities against mankind. However, his brother Alduin's claim to godhood caused him to rebel against him, siding with the humans by teaching them how to use the Thu'um.

In the Dragon language his name is spelled using the words for Ambition/PAAR, Overlord/THUR, and Cruelty/NAX

As with all Dragons, the lust for power was innate and still strong within him and if left unchecked, he would surely revert to his old ways. To overcome this, he retired to the peak of the Throat of the World and lived in seclusion so that he could meditate and focus. Although he played a big part in Alduin's eventual defeat, he was not happy about it, as he still considered him his brother and would mourn his passing.
paarthurnax