Dragonslayer

Alduin's Mist
Upon leaving the Hall of Valor and crossing the bone bridge, the next objective is to get rid of the mist in the valley. The Clear Skies shout needs to be used three times as Alduin will re-summon the mist twice.

Felldir: We cannot fight the foe in this mist! Gormlaith: Clear Skies - combine our Shouts! Alduin: Gormlaith: Again! Felldir: We can shatter his power if we Shout together! Alduin: Ven Mul Riik! Hakon: Does his strength have no end? Is our struggle in vain? Gormlaith: Stand fast! His strength is failing! Once more, and his might will be broken! Felldir: His power crumbles - do not pause for breath!

The three heroes will use the shout as well. After Alduin summons the mist again, the heroes eventually say to use it again. Alduin will now summon the mist for the last time. After a small dialogue, using the shout for the third time will get rid of the mist and Alduin will appear.

Alduin's Fall
Now that Alduin has appeared, he must be killed.

Tactics for defeating Alduin
The Dragonrend shout can be used on him as soon as possible to get him on the ground, where he can be attacked. Dragonrend can be used repeatedly to stun him and keep the other heroes alive. If he can't be killed before he defeats the other three heroes, lots of healing potions or spells will be needed to stay alive, as his fire breath attack is quite strong.

A simple tactic to defeat him is to use the same tactic used in the fight at the Throat of the World. A shield bash/stun will stop him from using his fire breath along with using the Unrelenting Force shout, and in between shield stuns, it should be possible to get two to three swings with a weapon in. Between using Dragonrend and stunning him, it should be a simple matter of ensuring that health doesn't drop too low from his physical attacks, plus maintaining stamina to use the shield bash.

There are also several unorthodox strategies to defeating Alduin. The player can wait on the sidelines using Dragonrend from a distance; as the heroes are essential and very powerful they can finish off Alduin after only a few minutes. However, once defeated, the player must deliver the final blow himself/herself. Alduin will watch the player, but not resist his fate.

Baiting Tsun into the fight is also useful. Stand behind him and wait, a meteor will eventually strike him and he will become enraged and join the battle. He is essential and will only fight for a short time before returning to his post.

For players using the legendary or master difficulty settings the two previous strategies are very helpful.

Tsun: That was a mighty deed! The doom of Alduin encompassed at last, and cleansed is Sovngarde of his evil snare. They will sing of this battle in Shor's hall forever. But your fate lies elsewhere. When you have completed your count of days, I may welcome you again, with glad friendship, and bid you join the blessed feasting.

All: All hail the Dragonborn! Hail him/her with great praise!

Tsun: Return now to Nirn, with this rich boon from Shor, my lord: a Shout to bring a hero from Sovngarde in your hour of need.

Quotes

 * I have already defeated your friends once. Beyne (Scorn). I do not fear them.
 * I have already defeated your friends once. Beyne (Scorn). I do not fear them.
 * I have already defeated your friends once. Beyne (Scorn). I do not fear them.
 * I have already defeated your friends once. Beyne (Scorn). I do not fear them.
 * I have already defeated your friends once. Beyne (Scorn). I do not fear them.

Trivia

 * After Alduin appears, the Hall of Valor will be permanently locked and the Dragonborn cannot return.
 * There is a brief moment where the option to loot Alduin's body appears before he disintegrates, although he cannot actually be looted. It is possible, if Kahvozein's Fang is equipped in the right hand, to harvest dragon heartscales.
 * When Alduin dies, the Dragonborn does not absorb his Soul. It is absorbed up into the vortex high above Sovngarde.
 * Tsun or any of the lost souls met en route to Hall of Valor might join the fight against Alduin if they wander close enough or are struck with one of his attacks.
 * After the main quest has been completed, the spawning rate for dragons seems to increase by around 200% making fast traveling nearly impossible without encountering a dragon.
 * If Alduin is killed close to one of the sentinel statues near the the edge of the cliff it will shatter in the explosion.
 * This quest marks one of the few times that the Dragonborn encounters a location, but can't interact with it after the quest is complete. The first is Helgen in the beginning, although the village is still allowed to enter.

Bugs

 * Some of the optional souls may cause the quest to glitch, not allowing any of the other souls to be helped through the fog. This seems to happen mainly with the first Stormcloak met on the path.
 * If the Storm Call shout is used in Sovngarde the count down bar for shouting again becomes locked. This prevents the clear skies shout from being used to fight Alduin.
 * Returning to Skyrim can be made impossible. If Tsun is thrown to the giant waterfall by making him follow near the bone bridge, then talking to him so that he faces away from the falls, and casting two-handed destruction spells on him (with the Impact perk activated), he drops down below. If he is followed, the Dragonborn will die instantly.
 * After using "clear skies" the required number of times, Alduin still does not turn up.
 * This can be fixed by reloading earlier save files until he does.
 * After defeating Alduin, the heroes may attack Tsun, making returning to Skyrim impossible.
 * The only way to continue is to reload an earlier save before the battle with Alduin.
 * If the three heroes are provoked into attacking the Dragonborn after completing the quest while still in Sovngarde, then the Dragonborn talks to Tsun to learn the shout without the conversation being interrupted by the now hostile heroes, they will learn the shout, but become unable to perform any actions outside of pausing and switching camera angles.
 * This can easily be fixed by reloading an earlier save file.

Achievements
Drachentöter Matadragones Ammazzadraghi Драконоборец