A gdi superweapon developed in cnc 1. It reportedly blasted kane's temple of nod in the first war. In the 2nd war, its power increased a bit but wasn't important story wise. In the 3rd war, one blast can wipe out almost anything. and it looks pretty to see when it begins to vaporize and enemy position.
In Tiberian Sun, an ion cannon can destroy an obelisk of light in one shot. In Tiberium Wars, it can destroy the obelisk of light AND anything in a significant radius.
Not much is known about it's origin at this time, but the Ion Cannon (AKA Godsend) is a destructive, electricity-based weapon fired from satellites orbiting the Earth. It can be activated by either direct control from an advanced Communications center, or it can be controlled by a commando on the ground, using what is known as an 'Ion Cannon Beacon', which is a small piece of high-tech machinery that sends out a signal to the satellite once it has been placed and activated, essentially acting like the laser sight on a rifle. It is not advised to stick around once an ion cannon beacon has been placed, unless you are a NOD engineer, at which point you simply dis-arm the blasted thing.