The 
Zodiac Spear is the most powerful weapon in the game. It has 150 Attack Power (AP) which makes it more than twice as powerful as your next best weapon if you obtain it as early as possible in the Necrohol of Nabudis. It also adds +8 to your Evasion stat and has a 4% chance of performing a combo hit.
Note that in the newest release of Final Fantasy XII: The Zodiac Age, the Zodiac Spear is no longer available in the Necrohol of Nabudis. Method 1, listed below, will no longer work for obtaining the spear and you must travel to the Henne Mines to obtain it.
There are two different ways of obtaining the weapon:
Obtaining the Zodiac Spear using method 1 (traveling into the Necrohol of Nabudis and avoiding the 4 listed treasure chests below) allows you to obtain the weapon very early in the game. The weapon will only be available to retrieve if you DO NOT open any of the chests listed below:
Lowtown (Rabanastre)
The treasure chests outside of Old Dalan’s place.
Cellars (Royal Palace)
The treasure chests inside the Cellars of the Royal Palace in one of the first rooms that Vaan starts off in.
The Confiscatory (Nalbina Dungeons)
The treasure chests in the room where Vaan, Balthier and Fran retrieve their missing equipment.
Vaddu Strand (Phon Coast)
There is an area of beach that contains 16 treasure chests in a grid formation. Only one of the treasure chests is forbidden but it is best to just stay away from all 16.
Assuming that none of the treasure chests listed above have been opened, the Zodiac Spear will be retrievable within the Necrohol of Nabudis. You can obtain it any time after the events at the Tomb of King Raithwall, however, it is quite a bit easier to obtain when you reach the part of the story that leads you through the Salikawood.
In order to gain access to the Necrohol of Nabudis, you will need to travel to the Salikawood and defeat King Bomb in the Grand Bower zone. Navigate to the Salikawood section of the walkthrough if you would like tips and tricks on how to defeat King Bomb. The Necrohol can then be accessed on the west side of the Corridor of Ages area.
This zone can be fairly tough if you are attempting to obtain the Zodiac Spear during your initial travels through the Salikawood on the way to the Phon Coast (eventually leading to Archades).
The Baknamy enemies specifically can be quite troublesome as they will appear out of nowhere and they can see/hear/aggro your party from quite far away.
Either take your time defeating them and attempt to pull them in small groups if possible, or you can run straight through as quickly as possible to the proper location.
You need to travel west through the Hall of Effulgent Light, going around the corner to the south and down the stairwell into the Cloister of Distant Song. Go north east and up the stairs into the Cloister of the Highborn, travel east, then north, then west, circling around and into the little room with 16 treasure chests laid out in a grid (shown further below).
The treasure chest that should contain the Zodiac Spear, assuming a north/south/east/west orientation, is in the third row down, fourth column over as shown below:
Make sure that you do not have a 
Diamond Armlet equipped on any of your characters when you open the chest as this will change the contents of the chest resulting in you not receiving the Zodiac Spear!
If the treasure chest does not contain the Spear, it means that you may have made a mistake, potentially opening a forbidden treasure chest.
Congratulations! You now have the strongest weapon in the game!
You may want to take this opportunity to seek out the Baknamy Merchant before you leave. Check out the Necrohol of Nabudis side quest page for more information by clicking on the link below:
This method of obtaining the Zodiac Spear allows you to obtain it later on in the game if you made the mistake of opening one of the forbidden treasure chests.
You need to obtain at least 10 Espers (check out the Esper section for more information on how to obtain Espers), complete Hunt 15: A Mine Flayer (Mindflayer – Rank IV) and speak to Geomancer Yugelu in Jahara to unlock the Phase 2 Dig area. You must be wearing the Diamond Armlet in order to have a chance to obtain the Zodiac Spear.
This method can be extremely frustrating in the original version of the game as the treasure chest only has a 10% of appearing, then there is a 10% chance of it containing an item and a further 10% chance that the item is the Zodiac Spear.
The only way to circumvent these odds is to manipulate the Random Number Generator to guarantee that you receive a rare item. Check out the Rare Item Trick page for more information.
