DREDGE

Story-Related, Cannot be missed

The steps to obtain the final relic feel a bit easier now that we have already gone through the trials of Stellar Basin and Twisted Strand. The main pursuit of Devil's Spine is getting three flames for the Fanatic that resides at the monument with the stone hooded figures and pillared dome.  Simply catch the below fish and mollusks and deposit them in the gray stone shrines around the steamy location:

Depositing the specimens in the proper shrines will give you a blue flame along with some research parts, trinkets and even two refined metals used for Hull upgrades. Add the three blue flames to the stone figures and the Fanatic will leave the Pocket Watch in your care.

Avoid the small Pirannha like fish during your time in Devil's Spine, Too many of them attached to your boat and they will summon the Unseeing Mother who can damage your boat pretty quickly.  These can be avoided by passing over the steam vents in the water. 

There is a secondary pursuit that involves finding three stone tablets in the area and bringing them to the Trader in Little Marrow. He will add them to one that he just happened to have lying around to make a large square tablet you can take to the ancient Lighthouse in the back of Devil's Spine to complete one of the final Pursuits and receive the Flame of the Deep lighting fixture that produces 3,500 Lumens! These stones can be found at the locations marked with crab pots in the below image (N14, P13, Q13) :

Returning to the Collector with the final Relic will not award you with a new ability. The Collector will invite you to travel out for one of the endings, so once you have fulfilled the Relic Pursuit you can wrap up anything you want before moving on to the ending, but there is no need to worry about getting anything completed before doing so, as it does not save your progress when you initiate either ending. 

Once you are ready, check out Unshackled  and Sated  for the spoiler guides to the two endings.

Retrieval of the relic and all Devil's Spine Pursuits for the Providence  trophy can be found in the video below. Please take note that the information in the below video was made before Patch 1.1.0 was released. The timer has now been removed from the Hooded Figure Pursuits, so you should have an easier time succeeding on that Pursuit: