The lighthouse door is always locked. The lighthouse keeper never lets anyone in... or so it's said. Hetman Arenim holds a spare key for emergency situations like these.

I trust you in dealing with those rogues, outlander.

What do you need the key for? Ivon Seloth has disappeared, and you think he was captured? Horrible! We will deal with these rogues -- whoever they are! You have my permission to enter the lighthouse, %PCName. Here's the key. An Indoril guard will meet you at the front door. He will accompany you in the raid. Rescue Ivon Seloth.