After obtaining the three pure metals, you can get the Phantom Sword and descend the Temple of the Ocean King to find Bellum.
Get the Phantom Sword
After getting the three pure metals, go to Zauz's Island in the Northwestern Sea and talk to Zauz. Then go outside and check the mailbox. You will get a letter from Jolene, challenging you to a duel. Talk to Linebeck. Say that you will accept her challenge.
Sail toward Jolene. The fight will be the same as before, except she will attack again after you knock away her sword. However, the next time you hit her, she will be defeated.
Then you can go back to Zauz's Island and talk to Zauz to get the Sword Blade.
Sail to Mercay Island and go to Oshus's house on the west side of the island. Talk to the old man to get the Phantom Sword.
Your next destination is the Temple of the Ocean King, where Bellum awaits.
Temple of the Ocean King B6
If you want to complete the temple with a perfect time, read the Temple of the Ocean King Perfect Time Guide.
Otherwise, if you use the golden light at the entrance of the temple, go north and go down the stairs.
Temple of the Ocean King B7
Step on the moving block, then when it goes left, step on the block above you. Face the left side so that you can block the arrows with your shield. When the block stops, walk onto the block on the left. From here, you can walk onto the safe area and use your boomerang to hit the gold pot to the right, giving you 30 more seconds. Then use the grappling hook to grab the treasure chest on the right. Open it to get the round crystal. The flames on the right side of the room will disappear. Go east onto the safe area.
Put down the round crystal and equip the hammer. Wait for the Swift Phantom to come near, then hit the ground near the Phantom with the hammer to make it come over and investigate. Then hit your safe area with the hammer to make the Swift Phantom come closer. Then when it turns its back, hit it with the Phantom Sword to kill it.
Pick up the round crystal and go east and then south, and go down the stairs in the southeast of the room.
Temple of the Ocean King B8
Go west across the invisible bridge, and carefully take the round crystal to the pedestal in the northwest part of the room.
The spikes will lower. Step on the switch to make the air vent turn on. Go down to the air vent and step in it to reach a gold pot to add 30 seconds to the hourglass. Then go down the northwest stairs.
Temple of the Ocean King B9
Open the nearby chest to get the triangle crystal. You can't go any farther, so take the crystal upstairs.
Temple of the Ocean King B8, part 2
Take the crystal east across the invisible bridge. Send a Bombchu through the hole in the wall to the north and make it hit the crystal switch on the right. Then you can take the triangle crystal northeast and down the stairs. (If you don't have any Bombchus, make your way to the northeast corner of the room and drop the triangle in the safe area there. Then go to the west and bomb the blocks to reveal the triangle pedestal. There is a safe zone under the block that you bombed, so stand there when the Phantom comes back. After that, go east to get the triangle crystal and take it west to the pedestal. The spikes down below will disappear, so go there when it is safe to do so. Hit the crystal switch to open the door that was blocking the stairs. Then take the triangle crystal northeast and down the stairs.)
Temple of the Ocean King B9, part 2
Use the boomerang to kill the invisible Wizzrobe in the northeast corner.
Take the triangle crystal to the pedestals in the middle of the northern part of the room. For now, put the triangle crystal on the ground.
Use the hammer to lure the Phantom to the pedestal area. When it starts to walk away, tap it to do a jump attack to kill it. Take the square crystal that it drops.
Now put the square crystal in the pedestal in the northwest part of the room (not the central part where there are three pedestals) to remove the flames. Go up the northwest stairs.
Temple of the Ocean King B8, part 3
Take the round crystal, then go south and walk onto the air vent to go up onto the wall. Walk north on the wall to reach the stairs that you just came up. Take the round crystal down there.
Temple of the Ocean King B9, part 3
Now put the three crystals into place, starting by putting the square crystal into the square pedestal, then putting the round crystal into the round pedestal, then finally putting the triangle crystal into the triangle pedestal. Then go north and you will feel a tremor. Go south again and you will be in a new area.
Temple of the Ocean King B10
Go northwest onto the safe area. Use the hammer to lure the Phantom that is carrying a key. You may need to go south from the safe area to make the hammer sound reach the Phantom. The Phantom can run past the spikes, so wait for it to reach the base of the steps below the safe area, then when it turns around, tap it to do a jump attack to kill it. Take the key.
Then go clockwise around the room until you reach the safe area in the middle of the southern half of the room. Tap the wooden post to stand on it, then use the grappling hook to grab the large treasure chest to the north. (If that treasure chest isn't there, you will have to go the long way into the middle of this room.) Then fall off of the right side, unlock the door, and go downstairs.
Temple of the Ocean King B11
Wait in the safe area and use the hammer to kill the Phantoms if you wish, then go east and down the stairs.
Temple of the Ocean King B12
Switch to the Hammer. Stand on the catapult, then wait for the Phantom to turn and go toward the left, and use the hammer to catapult yourself over the wall. Kill the Phantom but don't pick up the Force Gem. Instead, go north, staying near the wall, then use the hammer on the switch beyond the spikes to lower the spikes.
Go northeast to the chest in the safe area and open it to get a Force Gem. A Phantom will appear. Put down the Force Gem in the safe area, then use the hammer to lure the Phantom to you, then kill it as it walks away.
Take the Force Gem to the pedestal area in the middle of the upper half of the room, but don't put it into a pedestal yet.
Go to the northwest safe area and get the Force Gem from there. A Phantom appears on this side. Kill it like you killed the Phantom on the other side of the room. Take this Force Gem to the pedestal area, but don't put it in yet.
Then go south and get the final Force Gem and put it into a pedestal. Then go down the stairs north of the pedestals.
Temple of the Ocean King B13
Go northeast and pick up the red pot but don't throw it. Go to the left and fall down off of the ledge. Throw the red pot toward the Phantoms and step into the safe area. Then kill all of the Phantoms that appear.
After the Phantoms are dead, you can go through the door. Beyond the door lies the path to Bellum