Find 3 Soran'ruk Fragments and 1 Large Soran'ruk Fragment and return them to Doan Karhan in the Barrens.
When demons of the Legion fall out of favor, their spirits are imprisoned within orbs and distributed to their mages and warlocks.
Many were carried by their servants in Azeroth. Even fragmented, they still possess the powers of the demons trapped inside.
The orb of Soran'ruk was broken into two pieces, one was further broken to be used by the acolytes of the Twilight's Hammer in Blackfathom Deeps. The other, larger piece, was possessed by a wizard in Shadowfang Keep. Find them and I can reform it.