Leave Alinos and watch as your gunship picks up the weird object in the center of the cluster. The Oubliette is Gorea's prison, and we're about to make a house call. Before going in you might want to check the status of your inventory and Logbook. If you're going for 100% all around, by now your gunship should read 93% for Lore, 89% for Bioforms, 100% for Objects and 96% for Equipment.
You should also have only one Energy Tank left to find. If you're missing anything take a quick look through the Item Locations or the Logbook Guide for details.
Once you're all set dive into the Oubliette. Watch the cutscene carefully as Samus enters the void; you'll spot the other six hunters attracted towards it as well. Upon landing, take in the detail of this place, for a prison it sure is pretty (notice the decor is similar to the Stronghold Voids).
Exit the landing site and ride up the large lift you find. Peer through the observation window you find at the top, see the circular chamber beyond? Turn around and look for a teleporter, it'll take you straight there.
In this chamber you'll see the windows adorned with the eight Octolith symbols, each one representing an Alimbic Elder. Fire up your Scan Visor and look for a whole pack of Alimbic Lore here, including Sealing Gorea 01, 02, 03, 04 and 05. From these you'll learn what the Alimbic Order did with Gorea.
Firstly, claiming that no weapon could destroy the beast, they ensnared it in something called the Seal Sphere, locked it away in this Oubliette, then used the Alimbic Cannon to send the Oubliette into an alternate dimension. Got all that?
All it really means is that destroying this monster before it can escape is your final task to honour the Alimbic Order's legacy (and to stop the other hunters from making a seriously messed-up mistake - they really should've paid more attention to all that Alimbic Lore lying around).
Exit this chamber to discover another enormous chamber lined with rock walls and twisting vines. Look closely at the vines, see how they look large enough to squeeze the Morph Ball through?
Look for a narrow tunnel entrance near the door and sneak through with the Morph Ball. Follow the tunnels along to eventually reach a ledge overlooking the chamber down the far end. See the teleporter atop a floating platform nearby? Jump in to find yourself in a tiny room home to Energy Tank OB.01 - the final item in the game.
With your inventory as good as its going to get, take a quick look out the window of the chamber. See the giant twisting structure visible? You'll be finding out where that is soon.
Use the teleporter to return to the previous chamber. You might want to take a quick trip back to your ship now and save the game as there won't be any turning back soon. You should now have 100% of all Alimbic Lore scanned, so only a few bioforms left.
Exit your ship and head back to the large chamber with the twisting vines. Step forward and follow the path to locate a Stronghold Door. Open it and step inside the final chamber to discover a large circular portal in the floor. Take a deep breath and step into the blue light.
You'll now see a cutscene play as Samus enters Gorea's chamber. All six of the rival hunters have made it here before Samus and are busy tearing into the Seal Sphere at the center of the chamber, hungry for the power that lies within. Little do they know what it really is.
Upon breaching the seal a large beast will emerge, causing a moment's hesitation from each of the hunters. Before they can react eight tentacles swarm from the beast and ensnare them all, apparently sucking them lifeless.
With your rivals out of the picture the monster will turn its attention to Samus. So let's do this. You'll regain control in the giant chamber with Gorea awaiting in the center. Don't forget to scan the monster for info, and refer to the bosses guide for tips to find this thing's weakness and teach it to learn defeat.
But wait... that can't be it? Upon destroying Gorea the beast will erupt, setting off a chain reaction which ultimately results in the destruction of the Oubliette. Surely that means it's time to celebrate, but you'll never see Samus escape from the inferno. The game ends there with the hint that you haven't seen everything through to the end just yet. That couldn't be closer to the truth.
Remember all those Alimbic Prophecy scans you picked up throughout your travels? Fire up the game again and read them carefully in your Logbook. The first one mentions that the Alimbic Power will manifest when the "divine spectral sequence" appears. Each one then goes on to mention events taking place in sequence. Read them carefully though... each one hints at a specific colour.
Spoiler time... these colours represent the six weapons you've picked up in the game. When fighting Gorea you might notice a number of coloured symbols adorning the walls of the chamber high above. The Prophecy entries will give you the right order in which you should fire a weapon at the symbols to complete the sequence.
Follow this sequence (by shooting the coloured symbols with their corresponding weapon):
With the sequence complete you'll hear a echoing sound and your gunship will chime in warning you of an impending event. Defeat Gorea normally and watch as the objects surrounding the chamber pour energy into the beast. You'll be teleported to that chamber with the huge twisting structure you saw back when you picked up your last Energy Tank.
Gorea will now attack you in its final limbless form, but you'll first need something special to hurt the beast. Quickly descend the structure to the base, there you'll discover the seventh and final weapon of the game - the Omega Cannon.
This weapon may very well be the ultimate power that started this whole mess. Refer to the bosses guide to use your new toy to utterly destroy Gorea and add another race to Samus' life debt book.
With Gorea finished for good and the Oubliette history, you've done it! With the promise of ultimate power unfulfilled your rival hunters call it a day and head for the stars, but before joining them Samus receives a visionary thanks and a glimpse of the Alimbic Order's members. With their thanks delivered (and the lady under the suit revealed once more), sit back and enjoy the credits.
That's a wrap! Keep in mind that unlike before, Metroid Prime Hunters retains all your Logbook scans even beyond completion of the game, so if you haven't scored 100% yet check the Logbook Guide for entries you've missed. Now that you've conquered the single player adventure, why not hop on the Nintendo WFC and take the fight to other bounty hunters around the world?
See you next mission...