The Submerged Mystery trophy achievement involves capturing a giant fish. Now this fish is extremely rare and took me quite a long time for it to spawn.
It also requires you to pull off a small QTE which is quite easy but still be ready for it regardless. It also only spawns in one location and that is at Hoarfrost Reach and more specifically at area 8
Below is a quick guide on how to capture it. Also another quick note, you will need to fish it out rather than actually using the Capture Net.
SUBMERGED MYSTERY – Sealord’s Crestfish
LOCATION: Hoarfrost Reach. Area #8
HOW LONG DID IT TAKE UNTIL I MANAGE TO CAPTURE IT: Forever. 3-4 Hours.
Okay now if you have progressed enough during the story you should have a Campsite that is rather close to area #8, to make things more faster.
You should probably equip the Ghillie Mantle just in case the enemies here are still rather tough for you. Your choice. Anyway once you reach the fishing spot at area #8, you should hopefully find it swimming about with the other fishes.
If not you will probably have to farm some more, as I said it for me was a pain to spawn. So just leave and return over and over until it does finally show, nothing much else you can do really.
The fish itself is rather huge and has something dangling on top of it’s head. Just aim for this one, its so big it WILL stand out. Fish it to achieve this one.
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