Help Link and Linebeck defeat Bellum and restore peace to the seas.
Bellum's Chamber
Go north and the bridge will collapse behind you. Ciela will ask if you want to go back to prepare. A blue light will appear and you can step in it if you are not ready.
Go up the stairs.
Bellum, Evil Phantom
Bellum can spit slime onto the ground, and the slime can turn into eyes that attack at close range, so be prepared to avoid the slime balls, and hit the eyes with your sword if they appear near you.
Use the Grappling Hook to pull the slime off of Bellum. When it's all gone, use the Grappling Hook on Bellum's body to pull it close to you. Hit it with your sword until it is knocked back, then use the grappling hook on it again and keep hitting it.
After you do this a few times, Bellum will float up and latch onto the columns up above. Go up the spiral staircase, then use the bow and arrow to shoot the eyes on the columns. After you shoot them all, Bellum will fall back down below, so go down the stairs.
Use the Grappling Hook on Bellum to pull off the slime like before, and then grapple Bellum to pull it close and hit it with your sword again. Be sure to avoid Bellum's spit and kill any eyes that Bellum spits.
Bellum will float up again, this time spitting eyeballs at the stairs as you try to go up.
Shoot the eyes on the columns again, but this time Bellum will close some of those eyes, so keep moving until you find an eye that you can shoot. Watch out for drips of slime from the ceiling.
Next, Bellum will float up one more level. Shoot the eyes on the columns again, and watch out for when Bellum tries to slap you with eye tentacles or spit eyes and slime at you. Bellum might also float up and slap all of the tentacles on the ground, so stand behind a column to avoid this.
After you shoot all of the eyes on the columns, there will be a cutscene.
Ciela Powered Up
Afterward, go down the stairs. Ciela will explain what to do.
Take the Phantom Sphere that Ciela creates. She explains how to stop time: she says to tap the hourglass, then draw a figure 8.
Wait until Bellum's eye is pointing in your direction, then stop time. Then while time is stopped, run over to the eye and hit it repeatedly with the sword. But be careful not to wait too long before stopping time, because Bellum can float up to where you can't hit its eye with your sword for a while.
Wait for Ciela to generate another Phantom Sphere, then stop time again when Bellum's eye is pointing in your direction, and hit the eye repeatedly.
After you do this a few times, there will be a cutscene.
Bellum Ghost Ship
Shoot the eyes on the sides of the ship. The map shows you where the eyes are that are still open. Shoot the balls of slime that the Ghost Ship spits at you. Shooting the slime balls will sometimes restore your ship's hearts.
Shoot the eyes inside of the ship when you can. These three eyes are the main eyes that you have to kill to defeat the possessed Ghost Ship.
When you have killed all of the eyes, you board the ship, but you are asked to save the game first.
Bellumbeck
After a cutscene, wait for Ciela to create a Phantom Sphere. Take it, then tap the hourglass and draw a figure 8. Run around to Bellumbeck's back and try hitting it.
After another cutscene, circle around Bellumbeck and try hitting him with your sword, but don't get too close or he will swing at you. After you hit Bellumbeck a few times, your swords will get stuck together. Rub the stylus until you knock Bellumbeck back, then quickly hit him with your sword. When you are able to hit him, Ciela will be able to create another Phantom Sphere. Take it, then wait until Bellum's eye opens in the top screen. Tap the hourglass and draw a figure 8 while the eye is still open, Then run around behind Bellumbeck and hit the eye repeatedly.
As the battle goes on, Bellumbeck will sidestep after you hit him a few times with your sword. Be sure to run away from Bellumbeck to avoid his sword attack, then tap Bellumbeck after his attack so you can hit him again.
Later in the battle, Bellumbeck can also do a spin attack back and forth on the raft for a while. You can counteract this by doing a Spin Attack.
Keep following this strategy until Bellum is defeated. Be sure to keep watching after the credits roll, then tap the screen after "The End" appears on the touch screen. Congratulations!