Important: Evidently, you cannot advance any quests with Hack while you are on the Ghoulbane Empowered Quest. If you are on this quest, finish it first, then you can do the turn in at the end of this quest.
Click on the Treasure Chest underwater at ( -66, -8, 510 ) /waypoint -66, -8, 510.
Click on the Water Logged Scroll on the beach at ( -92, -3, 464 ) /waypoint -92, -3, 464.
Find the key by searching the beach for five clickable fish and move them to the basket of fish at ( 20, -3, 451 ) /waypoint 20, -3, 451. The fishes are not harvestable and not tabable. They will be along the shore or around the edges of pools of water, but not actually in the water. Moving the fishes will not break invisibility.
After moving all five fishes to the basket, you will receive the messages "A deep sounding tone echoes along the coast as the smell of rotten flesh drifts in the air" and "The Fathomless Horror voraciously devours the dead fish" to indicate that Fhoorl-Kurrat, a level 65 ^^^ abomination, has spawned and becomes attackable. Kill him to get the key.
Return to and open the treasure chest to receive the Hand of Glory.
Return to Havras to turn in the Hand get your reward and the next quest.