Yol Toor Shul - Fire Inferno Sun

Once the Dragonborn completes the quest "The Throat of the World," they will be allowed to Meditate on the Words of Power. If the Dragonborn chooses "Yol," a magic effect called "The Fire Within" will be added. It increases the strength of the Fire Breath shout by 25%.
When Paarthurnax teaches the player a Word of the shout, he says "Yol" in his dialogue even if the player knew Yol before meeting him and thus the Word learned was the second or third.
The Dragonborn is unable to learn the entire shout without joining the Companions, as the interior of Dustman's Cairn is locked until the quest Proving Honor is begun. However, if the Dragonborn used a soul to unlock a word of the shout before Paarthurnax can teach them like mentioned before, only two word walls are technically required allowing for the full shout without joining the Companions.