Fair Skies and Strong Winds

If Alex isnt at the Seat of the Aspects you probably have another quest on the go where she is elsewhere. On one of my alts she was still in Tyrhold, once I completed that she was back in Valdrakken

Added in patch 10.2.5 "Seeds of Renewal"
To start this little questline, speak with Alexstrasza at the Seat of the Aspects.

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  Fair Skies and Strong Winds
  
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Champion of the Dragonflights achievement and [title=821] account-wide title will be granted.

After giving you a recognition, new quests from Alexstrasza the Life-Binder and [npc=214051] will appear respectively.

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  A Place Beneath the Boughs and Siblings Until the End
  
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By doing the first quest, you will meet Tyrande and Shandris in Amirdrassil.

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  The Returning
  
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The rewards from completing this last quest are [item=212981] and [item=213004].

At first, these were the only rewards from the chain quest, but after a hotfix made 6 days after the initial patch, Champion of the Dragonflights achievement and [title=821] account-wide title were granted upon logging, by completing Fair Skies and Strong Winds

Other Night-Elf-themed transmog in Bel’ameth (Amirdrassil):
[item=213003]
[item=213005]
[item=213006]
[item=213007]
[item=213008]
[item=213009]
[item=213010]
[item=213011]
[item=213012]
[item=213013]
[item=213160]

All of these items are account-bound. If your character can’t wear some of these items, send them to your alts to learn the transmog.
No faction restrictions, you can fly to Bel’ameth and find these items without facing any hostile encounters as Horde.

/way Amirdrassil 54.69 77.20 Blue Kaldorei Bedroll
/way Amirdrassil 52.54 17.72 Blue Kaldorei Backpack
/way Amirdrassil 58.43 51.87 Night Elven Horn
/way Amirdrassil 49.14 70.33 Night Elven Signal
/way Amirdrassil 31.30 16.12 Kaldorei Bow Carver
/way Amirdrassil 48.28 76.40 Violet Kaldorei Pouch
/way Amirdrassil 55.30 64.31 Blue Kaldorei Pouch
/way Amirdrassil 47.88 56.85 Night Elven Shield
/way Amirdrassil 53.46 55.73 Night Elven Spear
/way Amirdrassil 51.89 5.89 Kaldorei Sentinel's Spyglass
/way Amirdrassil 29.04 28.83 Night Elven Bow

Just for lore and story, after completing Fair Skies and Strong Winds, you can find a conversation between Wrathion and Sabellian.

Sabellian says: Wrathion. Ebyssian mentioned that you will soon depart from the Isles.
Wrathion says: I didn't take you as one who engaged in gossip, Sabellian.
Sabellian says: Hmph. Does he speak true?
Wrathion says: He does. Iridikron may be lying low, but he still poses a significant threat to Azeroth.
Wrathion says: I will coordinate the Blacktalon's efforts to track him down. Even Iridikron can't evade them forever.
Sabellian says: Indeed. I must admit, your agents have impressed me far more than I expected.
Sabellian says: With rare exception, I never placed much stock in mortals beforehand. You have quite the way with their kind.
Wrathion says: I suppose I do...
Wrathion says: My time in the Dragon Isles was a grand adventure, but not the homecoming I'd hoped for.
Wrathion says: I've often felt that I share more in common with mortals than with dragons... and feared that I belong among neither.
Wrathion says: I thought that here, in the land of my ancestors, I'd find... something I was lacking.
Wrathion says: That by becoming an Aspect, I could prove who and what I truly am. If not to the world, then at least to myself.
Wrathion says: But I see now that I never had anything to prove. My place is among both mortals and dragons.
Wrathion says: That is my strength, not my failing. It's who I am. Who I've always been.
Sabellian says: Well said. You walk a path I never could, Wrathion. Take pride in it.
Sabellian says: And should that path lead you back to the Dragon Isles... I hope it does feel like a homecoming.
Wrathion says: Hah! We shall see. Thank you, Brother.

After today's hotfixes, provides the achivement Champion of the Dragonflights and the tiltle "Champion of the Dragonflights", as Alexstrasza called us during the quest.