The next upgrade is also in the lower section of Torvus, but this time it's in the Dark version. It's faster to start by going directly to the Catacombs, but we'll start from the Training Chamber in order to pick up a scan and an expansion on the way. The upgrade itself is more useful this time, since it can be used in more areas.
Get to the Training Chamber; the fastest route is probably through the Blue door in the Hydrodynamo Station. A new type of Blogg, the Darkling possessed version of the original, is swimming around here now; get a scan and defeat them with the Light Beam. You no longer have to mess with the Spinner here since you can just boost over the fence to get to the side sections. Go though the Black door.
Transit Tunnel East
As you pass through the first vertical pipe in the Morph Ball maze, notice the Bomb Slot at the top. You couldn't reach this before, but with the Gravity Boost you can do a double jump to reach it. This changes the water flow in the fourth vertical pipe which allows you to jump very high.
Drop a bomb to lift yourself up the pipe, then drop another bomb just before you reach the top so that you'll be coming back down when it goes off, giving a second boost. Do this again at near the top of this boost to reach another Bomb Slot at the top of the pipe. This changes the water flow again. Go back two pipes to the second one and do a similar bomb-boost chain to get to the top and pick up an Energy Tank. You have a time limit now though, and if you take too long you need to try again starting with the fourth pipe.
Continue through the maze.
There are now Grenchlers here, but they should be relatively easy to defeat in the water now that you have the Gravity Boost. There is a portal, but it's locked inside a cage above the water. Look under the cage for a Bomb Slot; you can reach it with a double jump now. Activate it to open the cage, then get the hologram scan and use the portal.
Dark Torvus Bog
Get a scan of the Darkling possessed Grenchlers before defeating them, then exit through the door here. You'll be going back the way you came only in the Dark Side.
There are pipes here as in the corresponding Light side room, but there's no water in them. You do have to time your way through some moving pistons though.
If you've been paying attention to the hint system you might think it's time to get the next upgrade now. Instead, you're only below the floor of the room where you'll be getting it eventually. You can see the boss you need to fight pacing around up there, and if you've got vibration on you'll feel it too. There's some rubble to one side and you might think you can climb up to the room above, but it's not actually possible. So continue through the White door.
The Morph Ball maze here is easy to get through, but there is a hidden Missile Expansion to get as long as you're here. First, drop down the steps to the lowest level and hop back up to the midway point and go through. Continue left down the steps but stop when you reach the bottom. Starting from the right end of that bottom section, do a hop to the left into a short section, move to the left and do another hop to the right to pick up the expansion at the edge of the safe zone. Drop back down and follow the steps on the left up to the exit.
Shoot the crystal to lower the force field so you get to the main part of the room. You've been here before, but now you can use the platforms you've just switched on to reach the White door near the ceiling.
Shoot your way through the tentacles to the other end of the passage.
Drop to the bottom, then get to the ledge around the central pillar just above. Now use the Orb Cannon to get to the top, and blast open the Red door here.
Save Station 2
It's probably a good idea to save because is nearly boss time.
Undertemple Shaft (cont.)
Drop to the grate at mid-height in the room, then down to the safe zone just under it on one side. Follow the short passage to a room with a dead Luminoth and activate the Bomb Slot there. This rotates the room and the passage so you can reach a Blue door; go through. Note, you can activate the Bomb Slot again to move the room back to its original position.
Sacrificial Chamber Tunnel
Shoot the containers for refills, and shoot the plants to make them retract.
Sacrificial Chamber (cont.)
Drop into the chamber to start a boss battle.
Pick up the Grapple Beam dropped by the monster; when you do, the floor lowers so there is no longer a passage underneath and the gate blocking the way you came in opens. To get out, scale the rubble on the side of the room, then grapple the hook on the ceiling to swing to the door. Get a scan of the hook before you leave though.
Undertemple Shaft (cont.)
Once back in the Undertemple Shaft, you can use the Grapple hooks to swing to the White door which leads to the Crypt and a portal back to the Light side.
Expansions (Torvus Bog)
There are two expansion you can get in Torvus Bog now, and a third expansion in the Temple Grounds.
Get to the Abandoned Worksite and get to the side with the door to the Great Bridge (assuming you didn't come in that way). Use the Grapple beam to get to the ledge near that door and grab a Missile Expansion.
Path of Roots
Going through the Great Bridge, get to the Path of Roots. Use the Grapple beam near where you came in to reach the top of the structure in the middle of the room and collect another Missile Expansion.
Expansions (Temple Grounds)
Path of Eyes
Get back to the Temple Grounds, but this time instead of taking the short cut through the Path of Eye, start going the long way and blast open the Green door about half way through.
Take a moment to enjoy the view, then use the Orb Cannon to shoot over the chasm. Try to avoid rolling into the opposite Orb Cannon when you land. Use the Grapple beam to swing over to the platform on the left and pick up the Energy Tank there. Swing back; the way forward is blocked for now so use the second Orb Cannon to get back.