Oh wow, I COMPLETELY forgot what the Rift of Aln skybox looked like.
https://wowpedia.fandom.com/wiki/Rift_of_Aln
Spoilers: it's filled with roots. Per old lore:
from which it is thought by the druids that the Dream itself originated
Gazing into it, Malfurion Stormrage saw it as a bottomless chasm which radiated with primeval energies that even he dared not investigate. Indeed, the very rift itself seemed half-dreamed
So it could be half Dream half Lifelands hence the funkiness, and red thorns/roots because Nightmare staining. Druids don't exactly know what it is, which ties into the idea that it could've been created by the Titans (pulling out Elun'ahir) and there's a conspiracy covering it up, hence why Elun'ahir is only a "legend".
Note that we also just got a Dream hearthstone expansion not about Amirdrassil, despite the look of the tree, but about SHALAdrassil and Xavius.
Who may not actually be dead at all per Sean Copeland.