This mob will only spawn when you deliver the proper item to Sliverlip Slim. Text of quest is pasted below:
Receive Package of Goods.
Deliver 1 Package of Goods to Silverlip Slim (Barren Coast)
Three a Blacksail brigand will arrive a few seconds later, running down the coast from the north. Two are human and one is a Troll. The Troll drops a Bill of Sale. These brigands do not see invis, and are snarable. If not aggroed shortly after their arrival at Silverlip Slim they will despawn.