Video Guide!
This is completed during the one last drink side quest, which can be started by talking to the captain in the dead swordfish in Thirdborn. The captain wants us to find his three former crew mates and bring them back for a reunion. So just follow the objectives to find and talk to all three crew mates. Two of them will agree while the third Ruanga will refuse as she believes the captain wants to poison her.
After speaking to all three, we are directed back to the Captain by the objective, but instead, we want to find the Captain’s living quarters up some ladders above the docks in Thirdborn. In his living quarters is a lockbox we can unlock with the use of three lock picks to find the poison he is trying to use. As a heads up, this area is a secret and is not marked by the quest at any point.
With the poison we can return to Ruanga, the crew mates who refused, and tell her about the captain’s plan to poison them. During this conversation, we want to choose the options to poison the captain instead and tell Ruanga to trust us.
Now we can return to the Dead Swordfish to wrap up the quest. As we enter, we will be handed some special drinks to bring upstairs to the captain. Once we reach the captain, make sure to select the option to switch the cups to poison him and pop the achievement