A Place Beneath the Boughs

If you are having a problem finding it, like I did ... In Ohn'ahran Plains, instead of going through the portal to Amirdrassil, go to the left or right of it and keep flying straight west till you get to the quest. Hope this helps.

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].

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

To tie together the posts made previously by cococat and skipn26, if you are starting this while IN the Emerald Dream, fly to AND THROUGH the portal at 72.7 53.0, which will take you out to Ohn'ahran Plains. Immediately do a 180/U-turn and then fly back West and go over, left, or right of the portal to head to Amirdrassil where you will find Shandris Feathermoon at 51.0 60.5. That all said, if you are heading there from Ohn'ahran Plains to begin with, then fly around the portal.

Caveat: If I knew how to post the coordinates I shared as links to the proper area versus just generic text, I would have. :)