The Executioner Class Battleship is a Cybran naval unit. It's armed with indirect-fire weaponry, anti-air cannons, and torpedo tubes. This unit can walk on land, with the LEGS Land Emergence Galleon System technology, which can be accessed through research.
The Executioner's proton cannons are a rather wonderful weapon. Their basic range is good enough, but with the Cybran naval range research upgrade, they essentially become mobile artillery, far out ranging even Fatboys. They fire in a low arc, and travel reasonably quickly, meaning they can shoot over small undulations in terrain (combining well with the Executioner's size, especially on land), but aren't too inaccurate.
Executioners make brilliant defensive units. Building a Naval Factory behind your base, then sending Executioners to the fortifications in front of it, allows them to cover almost all of your mass extractors, absorb firepower (especially when shielded) and blast away at enemy patrols long before they reach you.
Executioners are very large and dangerous-looking, and will be blasting holes in an opponent's land force long before anything else in your army opens up. As such, they will probably be perceived as a threat and targeted ahead of potentially more dangerous units such as the Bomb Bouncer or Megalith II. The Megalith in particular is actually dwarfed by Executioners; on land, they are almost as large as Cybranasaurus Rexes. Putting the Executioners in the vanguard of your army may allow them to draw large amounts of fire.
Best of all, these intimidating and long-ranged units require very little research to get hold of, and only a little more to bring onto land to supplement your Loyalists and Cobras.
Since Battleships are now much cheaper, faster access than in FA and SC1, they therefore have much less health (about five times less), so don't just build one or two even though they can still match minor experimental firepower.
With the Infinite War DLC, you can purchase short range Jumpjets for all non experimental naval units making The Executioner one of the most versatile ships in the game. Jumpjets could be used for moving ships over shores they cant climb or higher platforms in the back of a bases on some maps.
Beware that when on land, these ships wont stand much of a chance in close range combat as they only have long range weapons. Executioners are better suited at destroying everything before it ever gets close enough to shoot back.
The Executioner Class has surprisingly good Anti-Aircraft damage, being more powerful than both the Salem and the Poseidon and almost as powerful as the Mastadon.