The Zodiac is available for 1,500,000 bolts upon reaching challenge mode. Unlike most other weapons in the game, it cannot be upgraded in any way.
The Zodiac is a wrist-mounted orange weapon with small, blue lightning generators at the front, from which its blast is fired. When fired, it charges up for a brief moment before the entire screen goes completely blank, after which every enemy is destroyed.
The Zodiac is an immensely powerful weapon against all non-boss enemies. Its ammunition is limited and pricey at 10,000 bolts each, though it can be found in ammo crates. Ultimately, the player is likely to have far more cost efficient ways to destroy enemies by the time they can afford the Zodiac, and normal enemies are not likely to pose a threat. The RYNO II is a far better purchase, as its ammunition is less expensive, while it is just as powerful against normal enemies and far stronger against bosses.
Behind the scenes
The Zodiac was more specifically referred to as the Zodiac Mk.II. This is because the original "Zodiac" weapon was in Insomniac Games' first game, Disruptor, which also wiped out enemies within sight.