P.S. This is a solo quest, you cannot complete it in a party.
Belin (Saint's Haven)
Desolate Isle (Port Hermalte)
1. After defeating the Boss, talk to Dorin (Desolate Isle) and you will obtain a "Poorly Written Cryptogram".
2. Bring it to Stashy (Saint's Haven) to decode it. Then talk to General Duglas and Lena (Royal Castle) to complete the quest.