After recovering an Elder Scroll from Blackreach, the Dragonborn used it atop the Throat of the World. The scroll opened a window into the memory of Felldir's battle against Alduin, where the Dragonborn learned the Dragonrend shout and witnessed the events that brought Alduin into the Fourth Era.
Later, when the Dragonborn entered Sovngarde, Felldir assisted in defeating Alduin.
(Hakon One-Eye runs up to the summit of the Throat of the World to find Gormlaith Golden-Hilt waiting for him there. Dragons are seen circling the summit, roaring and strafing with their Fire Breath.) Hakon: "Gormlaith! We're running out of time! The battle..."
(He is interrupted as a dragon lands to confront him and Gormlaith) Dragon: "Daar sul thur se Alduin vakrii. Today Alduin's lordship will be restored. But I honor your courage. Krif voth ahkrin. Die now, in vain." Hakon: "For Skyrim!"
(Hakon draws his Ancient Nord Battle Axe and engages the dragon, soon joins by Gormlaith as she draws her Ancient Nord Sword and manages to get the dragon in a Kill Camera finisher as she mounts its head and delivers the killing blow) Gormlaith: "Hyah! Know that Gormlaith sent you down to death! Hakon! A glorious day, is it not?" Hakon: "Have you no thought beyond the blooding of your blade?" Gormlaith: "What else is there?" Hakon: "The battle below goes ill. If Alduin does not rise to our challenge, I fear all may be lost." Gormlaith: "You worry too much, brother. Victory will be ours." Hakon: "Why does Alduin hang back? We've staked everything on this plan of yours, old man." Felldir: "He will come. He cannot ignore our defiance. And why should he fear us, even now?" Gormlaith: "We've bloodied him well. Four of his kin have fallen to my blade alone this day." Felldir: "But none have yet stood against Alduin himself. Galthor, Sorri, Birkir..." Gormlaith: "They did not have Dragonrend. Once we bring him down, I promise I will have his head." Felldir: "You do not understand. Alduin cannot be slain like a lesser dragon. He is beyond our strength. Which is why I brought the Elder Scroll."
(He pulls out the Elder Scroll to prove his point) Hakon: "Felldir! We agreed not to use it!" Felldir: "I never agreed. And if you are right, I will not need it." Hakon: "No. We will deal with Alduin ourselves, here and now." Gormlaith: "We shall see soon enough. Alduin approaches!" Hakon: "So be it."
(Alduin arrives at the summit and touches down on the Word Wall to confront the three Nord heroes) Alduin: "Meyye! Tahrodiis aanne! Him hinde pah liiv! Zu'u hin daan!"
(Alduin unleashes his meteor shower shout) Gormlaith: "Let those that watch from Sovngarde envy us this day! Joor... Zah...Frul!"
(The three heroes unleash the Dragonrend shout on Alduin, forcing him to land and remain grounded because of its effect) Alduin: "Nivahriin joorre! What have you done? What twisted Words have you created?! Tahrodiis Paarthurnax! My teeth to his neck! But first... dir ko maar. You will die in terror, knowing your final fate... To feed my power when I come for you in Sovngarde!" Gormlaith: "If I die today, it will not be in terror! You feel fear for the first time, worm. I see it in your eyes." Felldir: "Fo... Krah Diin!"
(Felldir uses Frost Breath against Alduin) Gormlaith: "Skyrim will be free!"
(Despite their best efforts to damage Alduin with their Ancient Nord Weapons, Alduin is able to get Gormlaith in a Kill Camera finisher with his jaws and flail her about in his mouth before tossing her lifeless body away near the Word Wall) Hakon: "No, damn you! It's no use! Use the Scroll, Felldir! Now!"
(Felldir retreats to a safe distance, sheathes his Ancient Nord Greatsword, and prepares to read the Elder Scroll as Hakon attempts to keep Alduin busy and distracted from seeing what Felldir is about to do.) Felldir: "Hold, Alduin on the Wing! Sister Hawk, grant us your sacred breath to make this contract heard! Begone, World-Eater! By words with older bones than your own we break your perch on this age and send you out! You are banished! Alduin, we shout you out from all our endings unto the last!"
(Alduin manages to get Hakon down on one knee, sees what Felldir is doing, and tries to stop him with his Fire Breath, but is unable to prevent the Elder Scroll's power from being unleashed on him.) Alduin: "Faal Kel...?! Nikriinne... (roaring growl) Felldir: "You are banished!"
(Alduin vanishes into the time stream, leaving Hakon and Felldir alone on the Throat of the World to look over the aftermath of the battle) Hakon: "It worked... you did it..." Felldir: "Yes, the World-Eater is gone... may the spirits have mercy on our souls."
Gormlaith: "At long last! Alduin's doom is now ours to seal - just speak the word and with high hearths we'll hasten forth to smite the worm wherever he lurks." Felldir: "Hold, comrades - let us counsel take before battle is blindly joined. Alduin's mist is more than a snare - its shadowy gloom is his shield and cloak. But with four Voices joined, our valor combined, we can blast the mist and bring him to battle." Hakon: "Felldir speaks wisdom - the World-Eater, coward, fears you, Dragonborn. We must drive away his mist, Shouting together, and then unsheathe our blades in desperate battle with our black-winged foe." Gormlaith: "To battle, my friends! The fields will echo with the clamor of war, our wills undaunted."