This is the final mission. Tastidian tells you to go to the Psy Academy to ensure that it won't side with the Spriggats. Once you meet and talk to Piter, you regain all your memories and become exhausted. Once you fall asleep in your room, a man named Dean Lukas will be in front of you and ask you whether you want to side with the Psy Academy or with Tastidian.
Siding with The Academy
This is the canonical ending, where you must seek to kill the Shadowling Queen, Nelis. This is extremely difficult, so it would be wise to train as hard as you can. Once you feel you are ready (it's recomended to train your troops to have at least 40 HP, maybe more) go to Somnus, where you must face Tastisdian. The first battle you have is with lots of Shadowling Soldiers and two Shadowling Guardians. The soldiers are pretty difficult to beat, but cornering them could help. You should try to take out the guardians as quickly as you can, because they are capable of doing massive damage. Once you've beaten them, you now have to fight Tastidian. He seems to have infinite health, but he doesn't. There are a bunch of shadowlings protecting him, which you should take out first. Once you've done that, keep as far away from Tastidian as you can while you heal. After that, try to consistently deal damage to Tastidian, because he heals every turn. His attack makes this pretty difficult to accomplish, since it damages all units surrounding him, but an effective tactic would be to have attacking characters have "turns", healing once they get too damaged to keep fighting and swapping with another. The next battle isn't nearly as hard as the last one. It only has some Phantom Armors and Energy Golems. If you have Darkling, a good idea would be to shadowport behind enemies and attack them for backstab damage. Area-of-effect attacks are also helpful. The next battle wont be as easy...
Fighting the Queen
Expect party members to die. A lot of them. Nelis is very fierce, and it doesn't help that there are energy golems and phantom armors protecting her. First off, destroy the energy golems, and then take out the phantom armors. While doing this, try not to get attacked by Nelis, as both of her attacks are insanely powerful. After that, attack her with your most powerful units and don't stop. If one of your more useful units is in danger of getting killed, have one of the more disposable guys take one for the team by taking a hit with Holy Blast. Basically, try to keep your healer, your main character, and main damage dealer alive until she finally goes down. And there, you won the game!
Siding with Tastidian
If you side with Tastidian, you must fight Dean Lukas. This causes Anya and Guy to desert you.
After the Mission
After this mission, you have won the game. You have the choice of either starting a new game, or a new game+. A new game plus keeps all your stats. Also, when you talk to Hellion about his chest, you get a hefty reward.