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 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)