The Zodiak spear is in one of 16 treasure urns in one room in the Cloister of the Highborn in the Necrohol of Nabudis (If you have the Diamond Armlet equipped you will recieve a Dark Mater instead so make sure you dont have it on). There are certain conditions that have to be met in order for that chest to be there though: There are 4 treasure urns in the game you must NOT open for the spear to appear in the Necrohol (You're probably thinking "Oh " -_-).

The first urn you must not open is the one directly opposite Dalan's house in Lowtown.

The second is in the cellar you enter to get into the Royal Palace of Rabanastre at the start of the game. Its the urn in the south-east corner of the room.

The third is in a place called the Confiscatory in the Nalbina Dungeons. It's a small room with a save crystal that joins the Arena and the Black Watch. In here there are 3 treasure urns. The one you must not open is the one in the middle.

The fourth and final forbidden urn is in the Phon Coast. Remember the 16 urns all grouped together in The Vaddu Strand. Well one is in their. I'm not sure which one so I recommend not touching any.

I wish I knew this when I played through it cos I opened every treasure urn I came across and therefore missed that particular opportunity to recieve the Zodiac Spear. Fortunatly though all is not lost if you also did not succeed in this because you can also aquire it in the Henne Mines in the Phase 2 Dig. Beware though that monsters here are off a higher level than the Necrohol of Nabudis. Its kinda hard to explain so I hope my directions to the urn are clear enough. When you enter the Phase 2 Dig from the Ore Separation go straight and take the first right that APPEARS on the map (there is a secret right before that, that leads to a dead end and isn't displayed on the map). Follow that path and there should be some steps along the way too (just to make sure you're on the right track). When you come out of this man made structure part of the tunnel to the next dirt path there is another SECRET right turn that you won't see on the map that leads to a dead end. Just go along the right wall until you find it. You've now reached it. There is a single Urn in here but it's not as simple as just opening it. You MUST have the Diamond Armlet equipped, then it's a question of probability. The chances of you actually getting a Zodiac spear from this is quite poor. First of all the chances of recieving gil from the urn is 90% while the chances of getting an item are 10%. However it doesn't end here. If you do happen to get an item the chances of that item being the Zodiac Spear are only 10% again. The other 90% gives you an elixer.

Hopefully this'll help you guys out. I have yet to wield this weapon myself, so good luck with it.