DREDGE How to Get the Secret Ending

To get the secret ending, first get to the point where you can get to the regular ending. When speaking with the collector, instead choose the dialogue tree asking where he got the book.
When you complete this tree, you should have the book in your possession. However, do not choose the “I’m ready” option.

Instead, leave Blackstone and head back to Greater Marrow. Speak to the lighthouse keeper and she should have new dialogue. Tell her when you are ready and then follow her directions. Sail to where the light is shining and THROW IT BACK.

This will get you the secret ending as well as the achievement “Sated”