Josip Duljaj was a Serbian operative who worked for First Elder and by extension the Syndicate.
Duljaj was First Elder's most trusted assassin, and tasked with finding and killing Syndicate traitor John Markic in Montenegro. During the operation, he and his men were confronted by a Djorjic Militant group led by Dario Vukjovic. After long talks, Duljaj offered Vukjovic an opportunity to join him if he found and killed Markic. Vukjovic complied, and as a result Markic took him back to America.
After Vukjovic refused an order the First Elder to kill Cigarette-Smoking Man and fled the country, Duljaj was tasked with finding and killing him. Duljaj tracked him down, but Vukjovic got the drop on him and shot him in the head. ("Blood Debt")