Brimstone Glow is able to be obtained after running Consus Cave. The boss will drop a purple item called "Soul Gaze". Use it to see a large hidden egg in the boss room, if you leave the dungeon before using it the description states that you lose it. So one time use only in Consus Cave. You can run it multiple times to get multiple Brimstone Glows provided you get the Soul Gaze drop. The egg will sometimes rotate between a few different locations but all seem to be in the boss room. Once looted, the egg will go to your Marketplace bag as it is a Marketplace item.
The Soul Gaze item appears to drop the majority of the time you run the dungeon but sometimes there will be no drop. Ildruin has stated the drop rate should be 100% but as of right now it is not.
Here are screenshots showcasing the item drop and looting the large dragon egg -
And here is what the dragon looks like -
Disclaimer: This is a day one guide sorry if it becomes outdated!