Ulk are usually docile creatures, but one of them has started attacking people. Someone must put a stop to this!
This errand is available after completing the main story.
Your target is northwest of the Deep Dark Wood, so Travel there (save and recover at the Waystone inside to be safe), then have Tengri drop you off on the northern or western side of the lake there to find the Ulk. The big moosey doesn't have any weaknesses, so buff yourself up and sick your strong familiars onto him. It hits hard, so be sure to keep an eye on your party's health, as Esther can't always keep up with it.
Defeat it and you'll complete the bounty hunt, return to the Swift Solutions to claim your reward.