Originally Posted by
WolfRider
My Theory:
[World Trees]
The very first World Tree is Shaladrassil.
A World Tree allows a connection between the physical world and the Emerald Dream, the plane of Life and Nature.
After the first Legion's invasion, Cenarius and Malfurion seeded another World Tree because Shaladrassil was corrupted.
They planted the seeds in Hyjal, using a branch from G'Hanir, and named the World Tree Nordrassil. The World Tree was blessed by the Dragon Aspects of Life, Nature and Time.
Nightelves tried to plant another World Tree in Northrend, Vordrassil, but it was corrupted by Yogg'Saron.
[Vanilla]
After the destruction of Nordrassil, the World Tree in Hyjal, Night Elves seeded another World Tree on Teldrassil. This World Tree wasn't blessed by the Dragon Aspects so corruption started to invade it through the Emerald Dream.
Malfurion and other Cenarius circle druids were fighting the "Nightmare" in the Emerald Dream and couldn't escape because there was no "blessed" World Tree.
[Wrath of the Lich King]
Loken, one of the Keepers, was corrupted by Yogg'Saron.
He made a deal with Helya and plotted against Odyn (the Prime Keeper) to take his place.
Loken was helping Yogg'Saron to break from its prison, so we killed Loken.
By killing the Prime Keeper, we triggered a security system calling the Celestial in charge of Azeroth, named Algalon.
Algalon's job is to check the sanity of the World Soul and the corruption over its surface.
We defeat Yogg'Saron but Algalon was already there, and after several tests he decided to push the big red button in Halls of Origination.
We stopped Algalon and proved ourselves worthy to defend Azeroth from corruption.
We met the Dragon Aspects of Life (Alextraza) and Nature (Ysera), who decided to bless Teldrassil, and by the way, freeing Malfurion from the Emerald Dream.
[Cataclysm]
N'Zoth asks Deathwing/Neltharion to use the Halls of Origination. The reason: the Emerald Nightmare started by Yogg'Saron is now handled by N'Zoth and grows well. If life is erased from the surface and the Emerald Dream resets it, the Emerald Nightmare will corrupt a good part of the planet.
We stopped Deathwing to use the Halls of Origination and deactivated the device of reorigination with Brann Bronzebeard's help.
[Mist Of Pandaria]
Later we find a captive Keeper: Ra (or Ra-Den) who was the Keeper in charge of South Kalimdor and the Halls of Origination. When we met him, he broke his chains and was on his way to use the device of reorigination, but we stopped him
[Legion]
The Emerald Nightmare, still empowered by N'Zoth and invaded by Xavius, completely corrupted Shaladrassil, the very first World Tree.
We managed to defeat Xavius and helped Malfurion, Cenarius and druids to cleanse most of it, but a strong corruption core remains in the Rift of Aln, a dangerous part of the Emerald Dream. Shaladrassil isn't safe and there's still work to do there.
[Battle for Azeroth]
Teldrassil is burned down, which means that the last World Tree has been destroyed.
Beside the destruction of the Night Elves capital and an obvious declaration of war, what does the destruction of the last World Tree means?
Answer: it closes the door to the Emerald Dream.
And we know that Cenarius Circle was still fighting the Emerald Nightmare, so... Malfurion could be trapped... again.
[Five TORCHES to LIGHT our PATH] : Torches burn stuff
[Its surface BLAZES bright, masking the SHADOW below] Teldrassil is burning, closing the Emerald Dream until another World Tree is seeded. But Ysera is dead or waiting into the Emerald Dream to spawn. So even if Druids found a valid branch to seed another World Tree, Ysera couldn't bless it.
The connection is lost, the Old Gods' corruption can grow, faster stronger harder better, and nobody will be able to fight it.
Say goodbye to the Emerald Dream. And to Malfurion (probably).
[Battle for Azeroth bis]
Uldir is a freakin' place. A Titan facility where they tried to study the Old Gods' corruption, then later used by Zandalari Trolls. That could be the reason why Zandalari (the original Trolls) decided to avoid the C'thraxxi's corpse at the bottom of the mountains. They might have seen that Old Gods' corruption couldn't be fought, only contained.
While the Halls of Origination were the device that could use the Emerald Dream to restore uncorrupted life on Azeroth, Uldir would be an attempt to destroy corruption into the World Soul.
Or it could be the first version of Halls of Origination, and the source of corruption in the Rift of Aln.
Now what if someone used the device hidden in Uldir? What if the Emerald Dream was released?
Back to the topic:
Teldrassil is not a blind target. Somebody might told Sylvannas to use whatever she found in Silithus (a shard of Sargeras' sword?) on the World Tree, instead of Stormwind.
It would have be better to use it against the humans, because it was the first objective, but something happened and plans changed.
Like Blizzard said in Blizzcon: the real question is WHY Hord did burn Teldrassil.
Since my theory explains what would happen if there was no World Tree, I think the burning of Teldrassil was plotted by N'Zoth. But who is the pawn that would lead Sylvannas to attack the World Tree?