If you don't want spoilers, don't continue reading below here.
This quest, takes place in the underwater area, to enter this area you have to collect first 18 Points of Interest in Rookgaard.
To enter the underwater just go to the beach in the city here:
And then swim into the water and step into the bubbles:
Once underwater, just follow the path until you reach a small bridge, there you have to levitate down.
Be careful because once down you might lure up to 3 or 4 sharks at once, depending on your level a teams is recommended to kill them fast.
Now, head north west and follow this path, you will find mainly sharks on the way, at some point you will se dead end with something blocking your path, there go north and go up the ramp. Some Quara Mantassins will be there, so be careful:
On the north east side of that room you will find a lever, just pull it:
Now the path is clear and you can continue, just go back to the dead end with the warning signal:
Put your self on the end, and move west, you will drop to the next level. Go west (some Sharks on the way) at the end of the path you will find the teleport to the final room.
On this room, you will find the underwater Boss (The Drowned God) it has huge amount of HP and very high physical damage defense, so bring lots of supplies because it will take your time to kill the boss.
In case of troubles there is a TP that will take you out, but you will have to come all the way back if you wanna try it again.
Once you kill The Drowned God, a TP will appear on the middle of the room, go inside and you will find the reward room:
After collecting your rewards, TP will bring you back to the city.