The Zodiac Spear is one of the more powerful weapons in the game, but there are some special requirements in order to get it. There are 4 treasure chests that you CANNOT open in the game, otherwise the chest with the Zodiac Spear will disappear. 1) Chest in Lowtown, outside of Old Dalan's place 2) There are two chests in the southeast corner of the cellar when you sneak into the palace (before you get the Goddess' Tear); do not open these. 3) All of the chests in the Confiscatory, which is where you get your armor and weapons back after being captured. 4) In Phon Coast, there is an area with 16 chests all near each other. There is one chest among them that can't be opened, so it's best to not open any of them, since they only have Gil anyways. If you don't open any of the chests above, then you'll be able to find the Zodiac Spear in a chest in the Necrohol of Nabudis. It's in the Cloister of the Highborn area of the dungeon.