Obtain 10 Smuggled Mana Cells and return them to Exarch Nasuun on the Terrace of Light in Shattrath City.
I have devised a plan to acquire a power source strong enough for the creation of our portal.
The ethereal forces at Bash'ir Landing, atop the northwestern Blade's Edge Mountains, have been smuggling mana cells out of Tempest Keep and transporting them to the Sunwell for Kael'thas.
They have hidden the cells, but there is a way for you to find them; the ethereal carry phase devices that you can use to see the smuggled goods.
Just beware the mana cells' phase wyrm guardians.