Titans are enormous 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. Most of the races themselves are broadly based on those found in Tolkien's middle earth, but in a sci-fi setting. Chaos are an exception, being closer to the forces of Satan with a little paganism thrown in for good measure.
During the Horus Heresy, a great rebellion against the Imperium led by Horus, the Emperor's most trusted general, a great many Titan legions joined the rebels, who were secretly being manipulated by the dark gods of Chaos. By the time it was clear that the rebellion was born of something much darker than mere displeasure with the Emperor's rule, it was too late to escape the distorting, twisted clutches of the Chaos gods. Many of the traitor Titans were possessed by daemons (sic), and in the millennia since Horus' defeat have become warped parodies of the glorious monuments they once were. Many have grown tails with vicious close combat weapons, or have had their armour plating mutate into incredibly tough, leathery skin or crab-like shells. Some have undergone still stranger and more disturbing changes, and are enough to give even the most battle-hardened warrior nightmares. Most of these twisted beasts were once Warlord Titans, as other types of Titan were extremely rare during the times of the Horus Heresy. The forces of the god Khorne also field a huge Daemon Engine known as a Lord Of Battles, which is in many ways similar to a Titan.
The forces of Chaos field the same Titans as the Imperium, as well as two unique types. These are the Imperial Titans;
And these are the special Chaos ones (no w/us as yet);