Since this comes up so often, here are the excerpts from Stormrage where Alexstrasza and Ysera blessed Teldrassil:
- Alexstrasza reached out and took up the archdruid in one palm. She then rose higher and higher, so high that the entire crown was at last visible. Malfurion, still clutching the small branch, shook his head in disbelief at seeing such a height. He could not have flown so high himself.
“I have come to a decision,” the Aspect declared in a booming voice. “Though I and the others did not bless this tree at the beginning, a blessing is needed now!”
She spread her wings wide and a glorious, warm glow radiated from her. The Life-Binder smiled down upon Teldrassil as she would have any of her children.
“Let this blessing touch Teldrassil and all upon it!” the red dragon commanded. “And let it create for us a new hope and a new beginning, also!”
The golden-red glow spread from Alexstrasza to the crown. With astounding speed, it continued down toward the trunk and then below beyond Malfurion’s view.
And with that…it was done. Teldrassil was not only healed…it was now blessed, if only by the Life-Binder. Still, that single blessing meant very much.
Alexstrasza circled high above the World Tree. The transformation was complete. The glorious glow muted, but did not vanish from Teldrassil.
“This is a day to be well-marked by all Azeroth!” The Aspects pulled their hands away. The fantastic blending of auras and moonlight continued around the newly married duo. “But, most of all, this is a day well-deserved for these two here! Tyrande Whisperwind…Malfurion Stormrage…we bless this union…and, as our gift…we also do something else this glorious day…”
The expressions of the two night elves indicated that they were as ignorant of what this meant as the rest of the audience. In response, Ysera and Alexstrasza gestured toward the direction of Teldrassil’s vast crown.
“This tree was born without our blessing…it has been cleansed by the druids and blessed by one of us, but a more special blessing shall now be made…let it now receive through both us our hopes for a world and a future of which we can all be proud…”
Following their direction, the couple faced Teldrassil’s center. The Aspects each stretched a hand that direction. Malfurion and Tyrande smiled in understanding.
From the two transformed dragons, there shot forth a wondrous light that flowed to the archdruid and the high priestess, then outward.
It washed over the entire crown in a mere heartbeat, then spread down through the branches to the trunk and quickly beyond sight. What was visible to the onlookers clearly strengthened, flourished. No one there could deny that Teldrassil was obviously stronger, more vibrant.
The moment swept over Malfurion. His people, his world, was safe. Teldrassil was purified.
Most of all, he was at last one with his beloved.
A sense of fulfillment rose up within him. The archdruid smiled.
There were gasps from many in the audience.
“Mal!” Tyrande blurted. “What are you doing?”
He suddenly realized that he glowed and that glow now touched Teldrassil in a manner much as had the dragons’ blessing.
The assembled guests peered around in awe as even the surface beneath their feet was transformed.
The glow around Malfurion faded, but Teldrassil remained illuminated. In that light, its leaves grew even more lush than when Malfurion had cleared it of the Nightmare’s taint. Huge, multicolored fruit blossomed from the branches and, after reaching a thickness akin to the largest of apples, began to softly rain down. Those assembled laughed with delight as they caught the fruit and dared taste them.
“Like the sweetest nectar!” Elerethe Renferal blurted.
“Have to admit, nothin’ like this in the Aerie,” Falstad rumbled as he all but swallowed one of the two he had procured.
Tyrande tasted one, then also smiled. “Mal…this is amazing…”
He looked deep into her eyes. “This is because of you…”
She flushed.
“May your life be as full of life as Teldrassil is now,” Alexstrasza murmured to the bride and groom as she and Ysera stepped further back.
TL;DR: This was re-confirmed in Chronicle: Volume III on Page 191:
- Dragon Aspects Alexstrasza and Ysera decided that Teldrassil needed to be guarded. They both gave it their magical blessings
