Jerhyn is a Diablo II NPC, and is the ruler of Lut Gholein, the town of Diablo II's second Act.
He's not a merchant and doesn't buy or sell anything, but he does give out a quest that must be completed to advance to Act Three. Jerhyn's location changes in an unusual way; he is standing near Warriv when a character first arrives in Lut Gholein, eager to give his welcome message. He remains there until a character talks to him, or until the Arcane Sanctuary quest is activated, at which time Jerhyn relocates to the lavish Palace in the center of town. Characters must talk to Jerhyn after defeating Duriel, before they are permitted to sail on to Act Three.
If an aged Jerhyn still rules the mighty city of Lut Gholein in Diablo III remains to be seen.