How to Complete the Avenge of the Drowned Quest in BG3

Make your way to the Grey Harbor Docks in the Lower City. If you have the waypoint, teleport there.

Continue south from the waypoint until you hear a melody. Follow the sound to the Water Queen's House.

Speak with Flood Tide Allandra Grey. She will explain that Holli was killed by a beast in the Grey Harbor.

When Allandra asks for your tribute, tell her you've brought your weapon.

Select the dialogue option, “I’m good at slaying beasts - maybe I could help you,” to accept the quest and its reward.

After the dialogue, return to the Grey Harbor Docks. You can use Speak with the Dead on Holli, but it won't provide new information.

Speak to the fishermen on the docks to learn the beast was sighted near Flymm's Cargo, east of the docks and south of The Blushing Mermaid.

Head to Flymm's Cargo. You can enter through one of the three lockable doors on the perimeter or a sewer entrance. The quickest and safest route is often the back of the warehouse.

Inside, you will encounter Worgs. Defeat them.

Move crates and trunks in the northwest corner of the lower floor to reveal the Basement Hatch.

Descend into the basement. Proceed south and then through the door to the east at the end of the hallway to find a large room.

In this room, you will find Redhammer the Deviser. Engage him in dialogue.

You can choose dialogue options to inquire about the submersible, attempt deception or intimidation, or directly confront him about killing a servant of Umberlee. Choosing option 4, or options 1-3 followed by a skill check, will lead to a fight.

Defeat Redhammer the Deviser. This is generally an easy battle.

Return to the Water Queen's House and inform Allandra Grey that the beast is dead.

The quest will be completed, and you will receive the Wavemother's Robe as a reward.

Before leaving, you can interact with the submersible in the basement. This will take you to the sunken Iron Throne. Be aware that Gortash will threaten to detonate the Iron Throne if you approach, so heed his warning for now.