The Eldar are a highly advanced and ancient race in games workshop's warhammer 40k game universe, who bear a remarkable similarity to a sci-fi version of Tolkien's elves (or even his Eldar). They are a dying race, who wander the galaxy in enormous spacecraft called craftworlds, after the near-destruction of their race at the birth of the chaos god Slaanesh. These ships are created using psycho-plastics, a unique and incomprehensible technology that uses mental energy to form matter into complex devices. At the most basic level, these devices are simply tools made from an unusual material. However, more complex devices such as spacecraft and the giant robotic Titans are much more than simple tools; they are made from a specialised psycho-plastic called wraithbone, which is far stronger than adamantium, and will grow to heal itself if damaged. However, the most remarkable property of wraithbone is that it acts as a store for the mental energy of the Eldar, meaning that the souls of dead Eldar can be housed within it. This means that Eldar titans are not just machines; they are living entities, crewed not just by soldiers, but by a host of long-dead souls.
Eldar Titans are tall, graceful machines, that rely more on their impressive speed and manoeuvrability than heavy armour. This does not mean they are weak; far from it, they are easily as well armoured as any Imperial battle Titan. They are also protected by a holo-field, or dathedi, a complex field that breaks up the image of the Titan as it moves. This makes the Titan incredibly hard to hit with most weapons, but means that it must move as much as possible to gain the full benefit of its defences. This suits the fast-paced fighting style of the Eldar well, so isn't much of a disadvantage.
There are three main types of Eldar Titan (no w/us yet):