Chapter 19: The Source
Samus stands before the abomination.
Down in the Impact Crater you'll find a Save Station in the same room as the teleporter, so save the game now before moving on. The next corridor yields a very strange sight, this place is teeming with Phazon concentration everywhere. This Phazon is extremely hazardous and even with the Phazon Suit you'll still take damage if you touch it so watch out.
Scan one of the creatures swarming around the room on the opposite side of the pool to reveal it as a Lumigek - a strange salamander-like creature that seems to live off Phazon. Cross this weird corridor and go through the red door at the end. Inside you'll find yourself in the gigantic Phazon Core, where strangely enough a few Fission Metroids are flying around.
You'll need to climb up the unusual terrain to reach the top of the room, first go over past the giant pillar in the center of the room to climb up. Use the floating platforms to reach the upper walkway, just don't fall into the Phazon pools below. Quickly outrun the Fission Metroids flying around as you climb up, they shouldn't bother you if you keep moving.
Up on the walkway is a door leading to a Missile Reload Station, reload if you need to before continuing down the walkway. Go around the bend to find more floating platforms leading over to the door way over on the opposite side of the cavern. Wait here for a few Fission Metroids to come, then blow them all up with a Power Bomb. Be quick though as more will appear seemingly from nowhere.
This cavern is filled with really weird terrain, the place is twisted due to exposure of Phazon. In the next corridor you'll find the ground lined with deadly Phazon, but there's a network of Spider Ball tracks above leading through the room. Use them to reach the other side of the corridor.
Thankfully this area of Tallon IV isn't very big, go through the door and brace yourself for the supremely ugly sight that awaits in the next giant room. Walk into the room to face a massive black creature hanging from the ceiling. The monster will sense your presence and open itself up before dropping from the ceiling, revealing it to be some sort of giant black crab.
This monster is the ultimate source of all the Phazon, it came within the meteor from who knows where and is responsible for spreading the Phazon all throughout the planet. This thing is the Metroid Prime (now you know where the name of the game comes from), and you're about to show it that it should never have messed with the Chozo and corrupted their planet for all these years. Refer to the bosses guide for some handy hints on wiping this hideous beast out.
The core essence of Metroid Prime about to reveal itself.
Crushing this monster in Subchamber Four causes it to roll over stupidly into a side wall and go crashing down a huge pit. You'll regain control of Samus, there's no turning back now so walk over to the edge of the giant pit. The camera will pan down revealing an almost bottomless pit below. Samus jumps down to the very bottom to track down what's left of Metroid Prime, but it still has one surprise left.
At the very bottom you'll find its foul remains gurgling in a corner of this deep room, a massive creature will then emerge out of the remains. This huge tentacled brain is the core of Metroid Prime, and its last hope of survival. This is the end of your journey, use tips found in the bosses guide to take this thing down for good and fulfill the Chozo's prophecy, once the monster's dead you'll be the true Entrusted One.
I won't give away the ending finale of the game (or the bonus ending you'll get if you find all 100% items and scan all 100% of your logbook), but once you beat the aberration sit back and enjoy the finale - it's a good one.
Finale
And that's a wrap for Samus' first 3D adventure, and what an experience it's been. With the huge walkthrough above you'll be able to track down all the items the game's got to offer, as well as finding all the scannable stuff for your logbook.
If you've just finished the game on Normal Mode, give Hard Mode a try (or Hypermode in the Metroid Prime Trilogy version), all the enemies have double the health they had on Normal Mode, so you'll be up for a real challenge - especially against the last few bosses of the game. Check the secrets and tips and game endings sections for more hints on the game including how to unlock the four Image Galleries.