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For those who just have to open every single chest available to them as they come along will miss this item, as the sidequest involves not opening specific chests in four specific places. The reward of doing this - the Zodiac Spear - is the most powerful weapon in the game, as far as attack power is concerned, so feel free to try it on your first playthrough. The places are:

1. In Lowtown, the first and all other times you are there, there is a chest outside of Old Dalan's place, across the street - don't open it!
2. When you break into the Royal Palace (before you get the Goddess' Magicite), in the Cellars are several chests. Don't touch the two in the south-eastern corner.
3. In Nalbina Dungeons, don't touch any of the chests in the Confiscatory where you get your weapons back.
4. Later in the game, on The Phon Coast, there's an area far to the east called The Vaddu Strand which has 16 chests in a square. One of these chests shouldn't be opened, so don't risk it by opening any.

If you've left all the treasures alone, the Zodiac Spear will be in a chest in the Necrohol of Nabudis. You can get there through The Salikawood after defeating the King Bomb optional boss. It has +150 attack and +8 evasion, like all spears.

You can also obtain the Zodiac Spear in a chest in a hidden area of Henne Mines, but it's very rare (.01% chance overall) with challenging, level 60+ enemies around.