Fog Horn of Guiding

Near the coast there is always a problem of fog, heavy fog.  So the mages of Weatherworn have created a magic item which can help clear the fog enough to allow ships to come into harbor even on the worst of foggy days.  The Horn, which looks like a giant, curled ram's horn, sits on a high spot in the harbor and on foggy days is activated to guide ships in.  One ship at a time will be locked on to by the magic once the horn is blown and guided unerringly toward the docks. Once the ship is docked then another ship is brought in.  This can only be done for a dozen ships before the horn has to rest about 12 hours, though usually by then the fog has broken up enough for other ships to dock on their own.  There is always six guards and a mage in attendance of the horn, with a small building nearby to allow them to rest in the warmth when the horn is not in use.