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: "Gormlaith! We're running out of time! The battle..." 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!" 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." 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: "Meyye! Tahrodiis aanne! Him hinde pah liiv! Zu'u hin daan!" Gormlaith: "Let those that watch from Sovngarde envy us this day! Joor... Zah...Frul!" 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!" Gormlaith: "Skyrim will be free!" Hakon: "No, damn you! It's no use! Use the Scroll, Felldir! Now!" 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! You are banished!" 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."