Siblings Until the End

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

I saw the Alexstrasza cinematic then took the quest to go to Tyrande in Amirdrassil. I can't see Vyranoth and get this quest/cinematic. Anyone experiencing the same?

As of Midnight pre-patch:

To get the portal you must complete up to chapter 6 in the DF storyline.

If Vyranoth isn't showing for you go to Tyrhold and pick up "Silver Mettle" from Nozdormu, which leads to a slow quest chain inside the tomb.
You will then go back to the spire and get "A dislocated disc" quest then go BACK to Tyrhold. You will get a quest called "Reforging the Tyr's Guard".

On your way to that quest you will have unlocked "The Land Beneath". THIS should be the actual quest chain you need to get Vyranoth to show up at the spire. Do the quest chain in the caves (it's slow) then when the quest chain leads back outside, go to the spire and you should see Vyranoth at the top.

You will unlock adventure mode, which is what triggers the portal to show for all your alts.