Cyan's ultimate sword skill is locked away in his troubled soul. A trip to Doma Castle will help him unlock it.
Go to Doma
There is an island west of the northeast continent. Put Cyan in your party and land there, and go into Doma Castle.
Take a Rest
Go north through the double doors of the Castle, and go northwest to find the double doors into the first room of the castle. Go into the first door on the left, then go through the door on the right. Walk north into the room with four beds, and say Yes when asked if you need a rest. There will be a cutscene, and then you will be in Cyan's Soul.
188. Critic/Alluring Rider
189. Pan Dora
Use the save point if you wish, then go north up the stairs, then go west and follow the path. When you reach the three doors, go through the one on the right. Then go through the door on the right and talk to the person from your party who is there. Then go northwest and go through the door.
Go through the door on the right, and use the save point if you wish. Then go north, then west, and go through the door on the left. Go down and talk to the person from your party who is there. Then go northeast and go through the door.
Walk up the stairs and use the upper door, then go through the door on the right, then go through the door on the left, then go northwest up the stairs and go through the door. Equip protection against Petrify. Go south, and try to go through the door in the middle. A battle will start.
Larry/Laragorn, Curley/Curlax, and Moe/Moebius
Start by using Ice or Water-elemental attacks on Curley/Curlax (the one on top). Then kill Moe/Moebius (the one on the right), but don't use Lightning-elemental attacks on him, because he absorbs those. Finally, attack Larry/Laragorn (the one on the left) with Fire-elemental attacks.
After the Dream Stooges are gone, go through the door.
193. Samurai (Weak against Poison)
194. Suriander (Weak against Pearl/Holy)
Go west to find a save point. Go west into the next carriage, flip the switch twice, then open the chest.
Go west again until you see another switch. Flip it. The book on the table says to memorize which chests are open or closed. (You don't have to, because it is on this page.)
Open the chest to the left of the book to get the lump of metal. Then go back and put the lump of metal into the open chest between two couches. Now it is safe to open the chest above the one that you put the metal into.
Go west to the next carriage.
First, get the two chests in this area. The first chest is to the left of the door, but on the other side of the couch. The second one is to the left of that, beyond a couple of crates. After opening the chests, flip the switches in this order: far right, far left, far right, second from right, far right, far left. Then go west along the bottom wall to reach the chests. Set up the chests like this:
Open Open Closed Closed Closed Open
Then flip the switch above the chests, and go west. The small car has a save point in it. Go west from there and enter the train engine car, then exit, and you will be in some mines.
196. Io (use Bolt Beam)
197. PlutoArmor/PlutArmr (use Bolt Beam)
Go south and follow Cyan. Soon you will realize you are going in circles. Go in the other direction and follow Cyan. Then you will fall, and you will be in Doma.
You will see a cutscene, and then a save point will appear. Use it, and then you can go around to different rooms of the castle to see some cutscenes. When you are ready for a boss battle, go into the throne room.
Wrexsoul is weak to Ice. Wrexsoul will use the move Zinger/Fury, which causes it to disappear and possess a random character in your party. There is no way to know which party member he possessed. You have to KO or Break the possessed character to make Wrexsoul reappear. Wrexsoul is accompanied by two SoulSavers. These enemies will come back to life if you kill them. Their only purpose is to prevent the battle from ending when Wrexsoul disappears. It is possible to kill both SoulSavers and prevent them from coming back to life if you use X-Zone/Banish, ending the battle without defeating Wrexsoul. But if you do this, then you won't get the Pod Bracelet/Guard Bracelet that Wrexsoul would drop when defeated. If you decide not to use X-Zone/Banish, you might want to leave one member of your party dead after making Wrexsoul appear, attacking Wrexsoul with your best moves each turn. Then when Wrexsoul uses Zinger/Fury again, he will only have two people in your party to randomly possess.
After the battle, there will be a cutscene. When you wake up, Cyan will have all 8 SwdTechs/Bushidos, and a new sword, Aura/Masamune. Go to the throne room to get an Esper.