The Energy Cannon Mark III is the fastest and most damaging weapon in the energy cannon family. If energy weapons are your thing, you can't do better than this!
How to Obtain
You can either try trading for this weapon in a market world, or craft it yourself. If you decide to craft it, you have two options: start with a regular Energy Cannon and use an Onyx Weapon Upgrade Kit, or start with an Energy Cannon Mark II and use a Diamond Weapon Upgrade Kit. Note that if you start with a regular energy cannon and use two diamond kits on it, you can obtain the mark III version without using onyx or the onyx-based upgrade kit. Since most people find that diamonds are much easier to acquire than onyx, many people use the diamond-kit method to obtain this and other mark III weapons.
How to Use
Drag this item to your hotbar, select it, and have at it! It's especially useful in getting the Ghost Hunter achievements, since ghosts and revenants are susceptible only to energy weapons.