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In Baldur's Gate 3, there will always be something for you to do. Whether this is a side quest, part of the main campaign, or just going on a random adventure with friends. Sometimes throughout all of these adventures, you'll run into a quest that is a bit complicated.
This can be said for the Avenge The Drowned quest. This is a side quest that you'll run into that doesn't necessarily take a long time to complete. However, if you don't know what you're doing, then it will feel impossible.
How To Start Avenge The Drowned
Out of all the Acts in this game, you'll begin this mission in Act 3 and wind up on the same path you take for the plot in the Iron Throne. You will be in the Lower City and ultimately, you're going to want to find Flymm's Cargo. On the map, it will be just south of the Blushing Mermaid and east of the Grey Harbour Docks. The only way you will get this quest, though, is if you have not completed the Iron Throne Prison.
Listen to Flood Tide Allandra Grey speak just south of the Water Queen's House. If you tribute prayers, then it will be well received, and you can proceed to ask questions about the deceased.
You will be told that it was a metal beast that took her life. You will then tell her that you are good at slaying beasts, and maybe you can help her. Once she tells you to slay the beast, you will ask where you can find it. She will tell you that they have yet to find where the beast hides, but maybe you'll be able to. You will then tell her you will see what you can do. The conversation will end, and you will officially have the quest. Go to your quests and find out what you must do. You will start by speaking with the Grey Harbour Fishers.
Investigate The Grey Harbour Docks
On the map, you will see the Grey Harbour Docks above the Water Queen's House, east of the Steel Watch Foundry. When you reach the docks, you will see Gowlan the sailor scrubbing away at the edge of the dock just past the stairs. When you approach him, he will complain about the stain on the dock. You will ask if it is actually blood and he will deny it.
You will then tell him that you're looking into the death of a waveservant and would like to know if he knows anything about it. He will tell you he's one of the ones who fished her out of the water. You will then ask what happened to her body. After he explains that she was taken to the temple, you will ask if he saw anything unusual. He did see "some sort of wake moving towards the docks." With no boats around, he isn't sure what it was, but he thinks it might have been something big moving in the water.
He will end with saying he thinks it was heading towards the building across the channel. You will then have the option to continue asking questions or leave. Once you leave, you can start heading towards the building he told you about. Your quest will update directing you to search Flymm Cargo for the beast's master.
Search Flymm Cargo
Directly across from the dock where you spoke with Gowlan, you'll notice the double metal doors are locked. You can use the "knock" ability to unlock the doors. You should then use an ability such as Greater Invisibility to go through the doors unnoticed. This may work better than Stealth Mode. Once inside, go directly to your left, and you will see a trap door underneath some crates. It will be just past the first 3 crates to your left. A successful perception will help with locating it. Move the crates and go through the trap door to the basement.
Straight ahead will be a door called the "Detention Block." Enter through here and head straight to the big sturdy doors ahead. Once you enter the Machine Parts Storage, veer to your left around the corner, and you will find a Subaquatic Dock door. Utilize your invisibility and head through that door. You will see someone guarding the dock ahead. Quickly go to the shadows on your right, then make your way quietly to Redhammer the Deviser on the dock.
You may need Redhammer's help to complete the Iron Throne part, so keep that in mind before initiating battle with him or turning him in. You may be able to complete the Iron Throne prisoner extraction first before returning to pick up the battle with Redhammer.
Defeat Redhammer The Deviser
Approach Redhammer and speak to him. Accuse him of killing one of the servants of Umberlee and tell him that they now want him dead. When he asks if you're there to do the deed, you will tell him that he needs to face justice. This will initiate the fight. He is pretty easy to defeat when you utilize your abilities and maybe even use your best wizard spells. Once you do defeat him, you can head back to speak with the Flood Tide Allandra Grey. When you tell her that it is done and the beastmaster is dead, she will praise you before rewarding you.
Should You Turn Over Redhammer?
If you went on with completing the Iron Throne prisoner extraction, you will return with Flood Tide waiting. She will be a bit upset that you were conspiring with Redhammer; however, she will then allow you to turn him over, so you can be rewarded. If you choose any other option other than turning him over, you will be confronted by not only Flood Tide but also others. While you could kill everyone and loot them, the gear you'd receive would be useless this far into the game. Being on the Waveservants good side may be beneficial down the road.
Once you complete this side quest and defeat the beastmaster, Allandra will reward you with Wavemother's Robe. This is a very rare clothing that is resistant to Fire and Cold damage. It also has watery rejuvenation; if the wearer is standing in a water surface at the start of their turn, they will then heal 1-4 hit points.