Devastation is the ultimate weapon of the Renegades. It gets available when you build the Devastation building.
More buildings of this kind you have, more powerfull the weapon will be. If you upgrade all the devastations on your private zone to level 2, the Devastation will be at it's full power.
You can use the Devastation from any Fleet available in the Hot Zone. Just click on the Fleet, them click on the target (can be a Fleet, a Planet or an empty region of the Universe) like in the picture below:
Devastation has not any range. As long as your target is on a discovered space, you can fire it. Also, be aware!. The Devastation affects your Fleets or our allied's Fleets if they are in range