Titans are 100-foot tall robotic war machines from games workshop's warhammer 40k universe. They appear in their 6mm scale epic series of games. They come in many varieties, which vary distinctly between the races. The races themselves are broadly based on those found in Tolkien's middle earth, but in a sci-fi setting.
The Titans of the human Imperium are diverse and powerful engines of destruction. They range in size from the 50 foot tall Warhound scout Titan to the enormous Imperator, which stands over 150 feet tall, and is the most powerful war machine ever to be deployed on a planetary surface. All are powered by plasma reactors, a form of fusion reactor that is dangerously unstable, and prone to exploding with utterly devastating consequences when damaged. However, this is the only way to provide enough energy to drive the myomer-like artificial muscles needed to move the enormous weight of the titans as well as power their devastating arsenals. Imperial Titans are armoured with an extremely hard alloy called adamantium, a type of ceramic armour called ceramite, and multiple layers of powerful void shields, a type of advanced energy field. They are organised into fighting groups called legions, consisting not only of anything up to, and in some cases more than, 100 Titans, but also infantry and armoured support, making each legion an army in its own right.
The Titan legions are operated by a branch of the imperial cult called the Adeptus Mechanicus, the tech-priests of Mars who fanatically worship the emperor of mankind as the machine god. Each Titan is not only a fighting machine; it is a walking monument to the power of the god-emperor himself. There is no higher honour to a tech-adept than to be allowed to serve aboard a Titan, as they are viewed as the ultimate in technological achievement, and to a religion based around technology this makes them the holy of holies, second only to God himself.
There are four common types of Imperial Titan: