To get this powerful Dragoon weapon you must raise a thief so it has a job level of 70-73. Then when you are at least level 42 go to the Saronia Catacombs and battle Odin and have your thief steal the Gungnir spear.

Using 'Mog Net' send 7 mails to another player and then send 4 mails each to the NPC 'The 4 Old Men' and 'King Alus'. The order which the mails are sent does not matter as long as King Alus is last. This will unlock a new dungeon which is found east of the floating continent. Take the Nautulus to this location and submerge into the coral when you see a shadow in the water. In the coral there is a cave which is guarded by an Iron Giant who you must defeat.