Archmage Angela Dosantos at Light's Hope Chapel in the Eastern Plaguelands will grant you Arcane Cloaking at no cost. You must be Exalted with the Argent Dawn.

Description

In the thick of Plaguewood lies the entrance to the dread citadel of Naxxramas. Previously, all attempts to enter had been stymied by the magical wards placed upon the rune portal. That is, until now.

We have devised a way to gain entry via a permanent arcane cloaking of sorts - an old cantrip of the Kirin Tor with a few modifications of my own. With that said, the cloaking is costly; however, your dedication to the cause of the Dawn is unwavering! We will waive all cost associations.

Completion

You will feel a slight tingle, <name>. That sensation means it's working. Once complete, you will be able to enter Naxxramas by stepping upon the rune portal in Plaguewood.

The entirety of this quest happens at Eastern Plaguelands