A curious interplanar liquid that holds visions of other worlds.

You reach inside the chest and pull forth a plain glass jar of translucent gray material. Strange, colorful images pass through the material as if it were displaying the dreams of another world. - Battle at Blacklake (RPA-ADV1) 

Gray matter is a viscous liquid first seen contained within a simple glass jar, the prize within the durgolium chest guarded by the Ogre Tog.

It displays dreamlike images in its form.

Upon exposure to the outside world, a strange event takes place;

As the chest pops open, the earth quakes and the sky crackles with strange purple energy. For a moment, you hear the screech of bats, and you smell a briny sea. - Battle at Blacklake (RPA-ADV1)

Tucking it into a chainmail sack made of the same metal from Tarik ends the phenomenon;

You place this curiosity in Tarik’s durgolium sack. Immediately, earth and sky are still. - Battle at Blacklake (RPA-ADV1)

If you have any gray matter in your possession, you may show it to the Starlit Door members assembled in the underground, secret library. Impressed, they will allow you to participate in their meeting.

Entering King Taron's chambers, you witness him standing on his bed holding a jar of the gray matter. He is preparing to enter the rift forming above him.

Once he enters the rift, it closes behind him. The Grayfolk wandering his chambers immediately dissolve, splashing gray matter in multiple directions. A small portion hits you, causing you to suffer in agony.

King Taron still has a jar of the gray matter in his possession as he battles on Dagger Peak.

Placing a sample of the gray matter on a pedestal in the bridge of the Gurlung Colossus, it lowers into the floor. It appears to have awakened the dormant machine, opening a rift large enough to enter.

Standing in the Klynoks Archives, you witness giant bats flying overhead, each carrying containers of gray matter.

Shelves and shelves of the gray matter can be found in the Endless Stacks. Each jar appears to not only hold the gray matter, but also to hold memories of various Ulos denizens.

If your chosen profession is herbalist, you will encounter Grepet while visiting Hornhelm. He gives you a choice of items he is carrying.

Choosing the durgolium sack, he slowly spoons out a tiny amount of gray matter. You can see strange visions of other lands, floating islands, and living mists, all represented in that tiny droplet.

Grepet forces you to swallow the strange fluid. Your mind is immediately overloaded with histories of another world and dreams of its people.

“Interesting effect. I would like to experiment further but it looks like your time has come.” - Grepet - Saurian Wasteland (GS-ADV5)