DREDGE

Once all five Relics are turned in to the Collector, he will tell you he is ready to take his book out into the ocean at night for a final ritual.  One of three things can happen at this point and two of the three choices will give you the "Bad Ending" and this trophy, the third choice requires talking with another NPC for the "Good Ending" and the Sated  trophy.

It is important to note that this can be done without any effect to anything else you are working on in the game as it does not save your progress once you make the final choice who to sail into the water with to bring on either of the endings, so feel free to do both endings back to back and return to whatever Pursuits you still need to finish, or upgrades or encyclopedia entries you still have left because it will all still be there for you afterwords,

End game spoilers from here on out for both trophies:

Once you turn in the Fourth Relic, you will have an option to ask the Lighthouse Keeper at the Greater Marrow Dock a question about the "Old Mayor" this will allow you to actually set up the "Good" Ending without even having to finish the final Relic retrieval (if you are speedrunning the game or decided to play through a second time for the other ending and have already finished the Providence  requirements) This will open up the chance to stumble upon an old castaway at one of the many campfire spots around the map (See Safe Havens  for campsite locations). Talking with the Old Mayor will give you a massive clue into the darker side of the story and will then give you a second conversation choice when you talk with the Collector next.

For this ending, you have two choices: You can ignore the paragrah above and bring the Collector the final Relic (The Pocket Watch) and when he invites you out for the final journey, you can choose the only option "I'm ready" and this will bring you into the ending moments of the game and you will roll the credits in ignorant bliss of the real identity of The Collector. This will give you this trophy.

Your other option is to talk with the Lighthouse Keeper, find the Old Mayor and then speak with the Collector. This will offer a second conversation option heading into the end game: "Tell me more about that book of yours." This will open up a short dialogue between the fisherman and The Collector and reveal The Collector's true identity if you choose to persist and take the book (You can always choose "Forget It" and the conversation will revert to the beginning again). You will then take the book from the Collector and can decide to still head out with him or bring the book to the Lighthouse Keeper for the good ending. If you choose to go with The Collector you will get the identical ending as above, just with a bit more knowledge. 

Either way, The Collector will board your boat and you will be asked twice if you are sure you want to make this journey because the game will not save your progress from here on out. You will then need to head towards the giant beam of red light in the ocean to the left of Blackstone Isle. It will also be marked on your map (G8), but here is a screenshot of the location (you kinda can't miss it):