A Glowing Key is a side quest in Dragon Age: Inquisition.
Acquisition
Find the hidden cabin located northeast of Small Grove Camp, southeast of the Morrin's Steps marker and northwest of The Stone Tree marker (centrally located between the three). Once inside the cabin interact with the locked door; note the door can not be unlocked at this time. From this point, the holder of the key known as "Rebel Mage" will spawn in southernmost part of the Storm Coast near Driftwood Margin Camp. Rebel Mage is encountered randomly in the general area. The mage will attack on sight; kill the mage and loot the Glowing Key to begin the logged quest A Glowing Key.
- Rebel Mage will not spawn until the cabin is discovered and the locked door has been attempted to be opened.
- It is possible to encounter Rebel Mage in other various locations around the Storm Coast.
Walkthrough
Cabin location
The cabin's location will be marked on the quest map after looting the Glowing Key. Return to the cabin and open the locked door to complete the quest.
The codex on the table just inside the previously locked door grants XP. In the basement there are two chests; one contains the 
Mighty Offense Tonic Recipe, the other contains 
Sigil of the Golem (if Trespasser DLC is installed, this may not be the case depending on one's difficulty setting).
Rewards
- 44 XP
- 40 Influence
Codex entries
Codex entry: Notes Found in a Mage's Hut
Companion quotes
Upon seeing the ghastly scene, certain companions will remark on the apostates' disturbing house.
Cassandra: "This place makes my skin crawl."
Solas: "This place is unpleasant."
Vivienne: "This place is horrid..."
Sera: "Haha... Creepy, yeah?"