Crossbone Isle (宝島 Treasure Island) is an optional and secret dungeon-style location in Golden Sun: Dark Dawn. It is filled with powerful and unique enemies and equipment, in addition to the two most powerful summons, within its ordered series of ten floors of puzzles, and is most notable for having the most powerful boss in the series that aspiring players can fight. Unlike other optional dungeons in Dark Dawn, Crossbone Isle is normally inaccessible until after players have defeated the game's final boss, although it can be reached before that point by means of the Endless Wall glitch.
During the game's epilogue (when Matthew is in Belinsk after defeating the Final Boss), an NPC at the docks will mention a "special whirlpool" which is rumored to lead to a certain island. The NPC will also mention that someone has been there, and that person is currently in Yamata City. After loading the cleared game file, head to Yamata and find the man that the NPC in Belinsk was referring to. He will board Matthew's boat after he tells of the location of the whirlpool. Upon entering the Crossbone Isle cave, the man from Yamata will leave the party.
Go through the first area and check the treasure chest down the stairs. It’s a mimic! Defeat it for a Psy Crystal. Go down the stairs now to find a room full of columns. You can grab the two chests in here for 220 Coins and some Quality Zol. Go back out now.
Take the upper stairs and run around the edge to find a chest with some Ginseng. Now go back down and jump to the rock column. Move it to the left. Now you can jump and reach the chest for a Mythril Silver. Move upwards now and move the next column to the right. Use the new path and jump southwest to the next column, pushing it to the left. You can now grab the chest for 110 Coins. Move the column to the right now with psynergy and take the left path to get over to solid ground. Exit out the south. Push the columns down to make a way out to the right and continue onwards.
Down here you can again go down the stairs to find a room of columns. Grab the chests here for some Demon Mail and 440 Coins. Go up the stairs now.
In this room, jump over to the obvious chest for 330 Coins. Now we have to use some smarts. Jump back to the middle blocks and use the left blocks to jump all the way to the double blocks on the left. Now jump to the right, down, right, down, right, down, down, and right to get to the rock column. Move it off the blue tile to open up the blue blocks in the room. This will lead to the exit and a chest with Rice Ration. Get back to the top portion and step onto the breakable rock, then use search and move the rock column off the red tile. This will let you get the Apple from the nearby chest. Now take the exit, push the columns down for an escape route, and go down again.
As usual by now, take the stairs down to find some treasure chests, so loot them for 660 Coins and some rare Orihalcon. Go back out and up the stairs now.
This room has the same rock columns and colored blocks as the last one, but this time you need to use the columns to get to chests. Start by moving the column in front of you onto the red tile. See how this raises the red blocks? Now you can jump down and use the red block on the right for 550 Coins. Now you can move the column right off the red tile to reach another chest for some Quality Zol. Now put the red column all the way to the left and jump over to the blue rock column. Put it on the tile, which opens up a way to the last treasure chest for a Plump Dumpling.
Now leave the rom and come back in. Leave the blue tiles up and go make the red tiles come up. Stay on the right side of that platform when you raise the red tiles. Now jump to the platform with the yellow rock column. Push it to the right. Now jump over to the green platform and make it raise the green tiles. Use the platforms that rise up to exit this room.
Go down the stairs and grab the two treasure chests in the bottom room. You'll get 880 Coins and some Dark Matter. Now head back up and enter the top room.
This room is full of breakable rocks that you can only jump on once. Take the left path for 770 Coins. You'll end up falling down, but just come back (there's no way to avoid it). Now jump to the right chest, using the path going right for some Dried Lizard. Now jump up, left twice, down once, and get on the platform. Now jump up all the way to the top. Now use the right set of rocks to jump down all the way and right onto another platform.
Now jump right ONCE and grip the column. Push the rock off the red tile and then jump left and up to the room to grab a Iris Robe from the chest. You'll have to drop down again, but come back. Now to exit just jump up to the top area like before, except take the left-most path down and left, then grip the column. Now use the left path downwards to continue on. As usual, push the rock columns down and carry onwards.
Go down the stairs and open the chest to fight a mimic. You'll get a Cookie afterwards. Now let's explore the upper room first.
Here jump up to the skull column. Moving this thing will start making the rocks explode, so we got to move fast to claim all the treasures. Move it to the left first, then jump over to the first chest for a Mist Potion. Now move up and get the second chest for Alastor's Hood. Use grip on the nearby column. Now you can exit the room and re-enter to reset the blocks, so do so.
Now just do the same thing on the left side. You'll find 990 Coins and some Orihalcon. Now use the grip and reset the blocks. Re-enter.
Now we're going to get to the room to the north. Move the skull left and use the rocks to jump all the way north, then grip the column in the middle. Jump north. This room has a very old skeleton who tells you about more treasure deeper. We'll get there, we'll get there. The treasures here include Worn Bow, a Cleric's Ring (Which nullifies the "curse" on cursed equipment), a Bloody Claw (cursed), and some Dark Matter.
Now head back and fall down to get into the lower room. There a chest to the south with 1010 Coins. Move upwards on the left-hand side and use Search to get to a room with a summon tablet, Charon, which requires 8 Venus and 2 Jupiter Djinn to use.
Now head back and move up the right-hand side, using Search to get to another room with a summon tablet. This room is guarded by a boss though, the infamous Dullahan, so save before approaching the tablet.
After defeating Dullahan, approach the Iris Tablet, then leave Crossbone Isle.
- Unlike its appearance in the first game, Crossbone Isle is shaped like a skull in Dark Dawn.