This is a rather obscure quest which can be started by entering the Frostflow lighthouse, located between Dawnstar and Winterhold.
You can find the key to the basement (and dungeon) above the fireplace in the starting area, next to the firebug in a jar. There are several notes and diaries to notify you of what exactly happened in the location.
The quest ends by killing the Chaurus Reaper at the end of the dungeon, but to get your reward, you need both the key to the upper area and Habd's remains, found in the boss reaper. The key opens the upper area of the lighthouse and allow you to open the locked chest. You can take the skull to the brazier and you'll be rewarded with Sailor's Repose, a unique perk that constantly makes your healing spells (restoration school) 10 % more effective.
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