Reward: 8,000 XP, Med Emitter, Precursor Tablet. Go to the cave in the southeast corner of the zone and speak to Nestor there. Hand over the five Nocturne Relics you've collected in exchange for the final Precursor Tablet.
Side Mission: The Argus Event
Reward: 10,000 XP, Med Emitter, Ring of Life. For finding all five Precursor Tablets, you will have also have completed this Side Mission!
Now go to Foreman Krag for the multi-colony meeting. After that, go to the Cyclone and speak to Tails. Once you leave for Nocturne, there's no returning, so spend the rest of your rings (saving some for Continues, of course) on items and the best equipment for every party member. When you're ready, tell Tails you're ready to depart.
You have to split up your characters into two teams, with Sonic and Tails being mandatory members of one team and Knuckles and Shade being mandatory members of the other. For this reason, we'll be referring to the two teams as Team Sonic and Team Knuckles. We advise that you stick Amy in Team Sonic, and Shadow and Cream with Team Knuckles.
Take the 6 Rings here, and after Charyb triggers the alarms, proceed to a fight with some Nocturne High Praetorians. Use POW Moves to defeat them, then go through the doorway here. Fight the Nocturne Decurion (again, with POW Moves), then head right to find 3 rings, and 3 more in the next room. Head around the corner and hit the switch in the next room, then Jump up to find a box containing a Med Emitter, Refresher Wave, and Cure All Spray.
Head back out now and go west to the door you just unlocked. Enter inside and Sonic asks they should rest a bit. The game switches over to Team Knuckles.
Follow the 5 rings here. That's really as far as this team can go now due to the firing of guns, so switch back to Team Sonic.
Take the 6 rings in this room, then use the springboard to the left to reach the upper ledge. Head through the door to the left to get outside, then head southeast. Fly to the ledge there for 2 rings and a Chao Egg, then Fly back to where you began and head all the way back to the outside ledge again. Go northwest this time and Fly to the ledge there. Interact with the switch here to disable the guns, then switch back to Team Knuckles.
Fly across the two lava pits, go right for 3 rings, take out the Nocturnes here, then Fly northeast to find a Chao Egg. Go right and Climb the wall there to the top. You can't win the battle here against the invincible Gizoid Centurions. Instead, focus on keeping your party's HP and PP high for now, because you'll fight them for real in a moment with whatever HP/PP you have left.
Head around the corner and go north up the slope. Go into the doorway and grab the 3 rings on the stairs. Interact with the disabled Centurion on the left, then continue left to find 3 rings. Interact with the Centurion on the left wall, then Fly up to the black/blue structure for a Chao Egg. Return to the center of the room and head down the stairs. Go between the brown stones to find another inactive Centurion.
Jump down by the springboard, then enter the room to the east. After you defeat the Gizoids here, examine the last Centurion to the right of the doorway outside.
Team Knuckles must now fight (and defeat) the Gizoid Centurions. Use water-based POW Moves on them and moves like Echidna Rush. When they're defeated, head through the door on the left and open the box for a Med Emitter, Refresher Wave, and Cure All Spray. Head back through the door and Fly off the edge to the southwest. Follow this path to a wall, Climb it, then grab the Chao Egg. Head right to the next doorway, and go through. Take the 5 rings here, go back through the door, and head back until you reach another wall you can climb. Heal your party, then climb the wall.
Head up the stairs and down the southeast hill to find a Chao Egg, then go back up the hill and go down the slope to the southwest. Dash at the bottom and take the Chao Egg at the end. Ascend the blue staircase, then Dash over the ramp to reach the middle of the map. Ascend the sparkling stairs here, heal your party, and go through the door.
Charyb floods the halls here, and the entire team's PP will drain every second they're underwater. Once a team member's PP depletes, that member's HP will decrease per second. Use the air bubbles here to restore your lost PP, and avoid the Nocturnes, since you can't use POW Moves or flee underwater. Move to the first air bubble, then the next. Continue down the hall to find a Chao Egg, then sprint back to the last air pocket. Enter the door on the right and follow the wall north into the next room. Use the air pocket here, then head right into the next room. Use the air pocket here to completely restore your PP, heal your party if you're hurt, then head up the ramp to Charyb. You can't win this fight, so don't even try. Just Defend and use healing items to keep everyone's HP up.
BOSS: Prefect Scylla HP: 700 • Weakest Against: Water
Prefect Scylla's attacks can deal over 100 HP worth of damage with every hit, so your healer's got to work overtime to keep the party healthy! Make sure to hit all the inputs when he tries to use Mighty Slash, since that attack can stun you. Use water-based POW Moves to deal the most damage.
After you defeat Prefect Scylla, you'll receive the Chaos Emerald 6. Now head up the ramp and enter the door to the left. Grab the rings and Chao Egg here, then head up the stairs and open the box on the right for a Med Emitter, Refresher Wave, and Cure All Spray. Now interact with the valve in front of the top stair to drain the water in the corridors where Team Sonic awaits.
BOSS: Prefect Charyb
HP: 700 • Weakest Against: Water
Defeat Charyb the way you just defeated Scylla, using water-based POW Moves. You may need to start out the battle healing your party if you got slammed in the previous unwinnable fight with Charyb.
Defeating Prefect Charyb nets you the Chaos Emerald 7. Now back to Team Knuckles...
Head up the ramp on the left of the doorway and Jump to the blue glass rooftop. Head northeast and Jump off the side there. Go right, grab the Chao Egg there, then head back and go up the steps to find 7 rings. Go back down, then Climb the wall after you've healed your party fully.
Head back through the door, then go south to the next room for 4 rings. Enter the next room to the right for 4 more rings and a Chao Egg, then go back to the previous room and take the elevator up to the top level. Head northwest for 4 rings, then go southeast and use the springboards to reach the last Chao Egg in the game. Jump down to the south to return to where you started, then return to the upper tier. Make sure everyone's healed up and that you have equipped your Chao and best armor and accessories, like the Angel Amulet. Now head to the large door marking Ix's chamber.
BOSS: Imperiator Ix Power Throne & Power Pylon x2
Imperiator Ix Power Throne - HP: 800 • Weakest Against: Fire'Power Pylon - HP: 300 each • Weakest Against: Water
Ix can call for more Power Pylons if you defeat them, so don't bother attacking them first; go after Ix instead! Ix is extremely powerful and can kill a character in two hits, so make sure you're healing copiously. Use Echidna Drop on him, then hit him with your most powerful fire-based POW Moves. If Cream is in your party, have her use Demoralize on him.
Imperiator Ix - HP: 800 • Weakest Against: Fire Gizoid Guardian - HP: 600 each • Weakest Against: Water
Before Team Sonic can fight Ix, they must first catch him. Jump over the large spheres in your way (hitting them knocks off 25 HP each). Once you catch up to Ix, the same tactics you used in the battle that preceded this one apply here. Ignore the Gizoid Guardians and focus all your attacks on Ix. Sonic and Amy's Fastball move works great here. Make sure you're countering his moves and curing anyone that gets Stunned.
Ix is not dead yet! Instead, he is becoming extremely powerful! Sonic has turned into Super Sonic!
Ix may be stronger with an obscene amount of HP, but then again, so does Sonic! That said, focus on countering his inputs during his round, and when your round comes up to use Super Sonic, make sure you hit all your commands. If you don't knock him out in one round, he'll have a large chunk of his HP restored to him. Counter his moves again, then try again to wipe him out in your next round to finish him off!