This page in our guide to GreedFall contains a walkthrough for The Trials of Water main quest.
Tell Constantin about En on mil frichtimen. Then, go to meet Gendan, the leader of the council. Ask for a meeting with the council, and the guard will announce that only friends of the council carrying a special seal can enter. To continue the quest, you need to complete The Suffering of Constantin main quest and gain Castach's trust. Go to the healer - he is with Constantin. Ask him for his seal.
Go meet Glendan, the head of the council. After the conversation, you have to complete The Trials of the Water. In addition, you also start Siora's side mission - Promises Set in Stone. Go to the specified location and enter the cave.
At some point, you will reach a room with a bowl of water. Your character will tell you about the vision, and you have to complete a puzzle similar to the previous one, the one invlolving elements. Nadaig, whom we killed, appeared before you were able to start solving the puzzle. That prevented us from completing the puzzle.
Move forward and approach the painting on the wall. Then, return to Glendan and tell him about the trial. This completes the mission - you get +2,000 XP.