It is explained the Paarthurnax committed several atrocities against humans during the Dragon Wars, but he eventually had a change of heart and defected to the human side, in which he taught them the power of the Thuum to aid in their battle against the Dragons. After Alduin was sent forward in time, Paarthurnax spent the next thousand years training those who wished to learn the Thuum and to medidate atop the mountain "The Throat of the World".
His meditations were necessary in order to suppress his natural desires of power and the urge to dominate, as it is explained that it these desires are natural in every Dragon.
When Alduin returned, Paarthurnax helped train the Dragonborn in order to combat Alduin. He also explains that there is a certain dragon shout that the Three Heroes used to defeat Alduin, through the use of an Elder Scroll. The Dragonborn, after recovering the scroll, used it to witness Alduins defeat and discovered the dragon shout "Dragonrend". Paarthurnax aided The Dragonborn in his battle, and defeated, but not killed Alduin, who had fled to Sovngarde.
After the Dragonborn finally defeats Alduin, the Dragonborn is then teleported back to The throat of the world, where Paarthurnax and several Dragons gather to announce Alduin's defeat. Paarthurnax then states to the Dragonborn that he will become the new leader of the surviving dragons, and promises to teach them his ways. Time will tell if he holds true to his promise.
Trivia:
- Paarthurnax was voiced by Charles Martinet famous as the voice of Nintendo's iconic character Mario.
- There is a side missions given by the Blades where they order the Dragonborn to kill Paarthurnax for his crimes. Interestingly, a vast majority of players were outraged by this command, since Paarthurnax became well liked by players for being the mentor and one of the very few friendly dragons who greatly helped them in their quest. Players went so far as to install mods which allowed them to permanently kill Esbern and Delphine for this demand.