Use the "Bend Will" Shout to tame Sahrotaar | Main story mode - At the Summit of Apocrypha TES V: Skyrim - Dragonborn Guide

After reaching the top of the stairs, you will be attacked by a giant Dragon - Sahrotaar. As he lands beside you, don't try to fight him, but instead use the Bend Will Shout. That way you will tame the beast and will be able to get on its back. Before doing so, approach the wall to learn one of the Words of the Dragon Aspect Shout.