Cauternus will approach the party when they first enter the Elfsong Tavern, and ask if they are monster slayers. No matter the answer, he will ask them to kill hobgoblins in particular.

Aye, you be fighters and mages and monster-killers all, be ye?
The roads be lined with bandits of every ilk and description but the most fearsome be them monstrous hobgoblins! Do us all a favor and place a blade into their gut!