The Triangle Crystal is an item from Phantom Hourglass found in the Temple of the Ocean King. The item appears in a Treasure Chest at the northwest corner of Floor B9. Link will need to carry the crystal back down to Floor B8, where it fits on a pedestal in the northeast corner, needed to reach a crystal switch. Placing it in this pedestal also allows Link to reach a Treasure Chest that contains a Courage Gem. Eventually Link will need to carry all three types of crystals and place them in corresponding pedestals on Floor B9.
On Floor B9 of the Temple of the Ocean King, if Link places the crystals in the proper order, the elevator will activate, allowing Link to reach Floor B10. The correct order is Square, Round, and then Triangle.
Each time Link re-enters the Temple of the Ocean King, the crystals will revert back to their original locations. In subsequent visits of the Temple of the Ocean King, Link will not to place the Triangle Crystal on Floor B8, as a shortcut with the Bombchus will skip this step.