Black Blood of Yogg-Saron

Yogg-Saron is described by the resurrected Ursoc as "the beast with a thousand maws", and is only the second of the Old Gods to be explicitly named as such. Apparently Yogg-Saron dwells deep beneath the Grizzly Hills of Northrend, where the roots of the failed World Tree Vordrassil have penetrated his lair. Through these roots Yogg-Saron corrupted the tree, as well as the Grizzlemaw Furbolgs that moved into its shattered stump.

Slinkin the Demo-gnome, killed while investigating the Wintergarde Mine, discovered before his death that the Scourge are apparently at odds with Yogg-Saron. His last note indicates that the word "Yogg-Saron" is said with great contempt and is usually followed by loud outbursts. The Scourge are mining Saronite in the Wintergarde Mine to fuel their war machines, implying that it is a material derived from Yogg-Saron. The alpha name (or at least icon file name) for Saronite is 'Yoggthorite' which strengthens the association with Yogg-Saron.

Yogg-Saron's underground lair might extend across the Dragonblight and the Grizzly Hills, if not beyond. He has even managed to corrupt Loken - a previous servant of the Pantheon - and his followers, that are located in Ulduar.
(source: http://www.wowwiki.com/Yogg-Saron)

Yog-Sothoth knows the gate. Yog-Sothoth is the gate. Yog-Sothoth is the key and guardian of the gate. Past, present, future, all are one in Yog-Sothoth. He knows where the Old Ones broke through of old, and where They shall break through again. He knows where They have trod earth's fields, and where They still tread them, and why no one can behold Them as They tread.
—H. P. Lovecraft, "The Dunwich Horror"

Entrance at approximately 26.9, 50.4
Just to clarify:
You are going down into a cave system, not looking on top of the words "Azjol Nerub" for quest items.

Also, they look similar to Thorium nodes, and are all along the walls. Small compared to the mobs, but shouldn't be too hard to find.