Imperial battle robots are maintained by the Legio Cybernetica, which is a fighting arm of the Adeptus Mechanicus in the same way as the Collegia Titanica. They are assigned to Space Marine forces for battlefield duties, but are not part of the Legiones Astartes.
Robots have seen service in just about every campaign of the Horus Heresy so far. Perhaps one of the most famous robot-vs-robot action took place in the Eastern Industrial sector of Myrdinn City during the Scouring of Estessian, when small but well-programmed forces of Traitor robots held off a small force of Firebrand Titans supported by dreadnoughts and robots for almost an hour, causing considrerable disruption to the planned flanking sweep.
The majority of Imperial robots which have seen service in the Horus Heresy belong to the following five classes:Crusader
The Crusader class was designed for speed and agility, and has been most successful in close assaults against infantry and other walkers. It has also been effective in the role of tank-killer, thanks to its speed and firepower of its back-mounted las-cannon. 4 hard points (light armour. fast. power sword in each arm, back mounted las-cannon which can be substituted for heavy bolter or melta-gun)Colossus
The Colossus was developed as a siege robot but has proven itself sufficiently versatile to take on a wide range of battlefield roles, including support and close combat. 4 hard points (armed with melta-gun on back which can be substituted for lascannon or heavy bolter, bolter in right arm which can be substituted for a seige hammer auto-cannon or power fist, and siege weapon in left arm which can be substituted for a flamer boltgun or auto-cannon. slow)Cataphract
This one is one of the heaviest of the Imperial battle robots, designed for a general-purpose role with the accent on versitility and durability. 4 hard points (armed with las-cannon on back which can be substituted for a heavy bolter or melta-gun, bolter in right arm which can be substituted for a power fist seige hammer or auto-cannon, and flamer in left arm which can be substituted for a power fist auto-cannon or siege hammer)Castellan
The Castellan class is a heavy assault robot, slower then the crusader but more hevily armoured. The most common configuration concentrates on anti-personnel weapons. 4 hard points (heavy bolter on back which can be substituted with a las-cannon or meltagun, power fist in both arms which can be substituted with a bolter seigehammer auto-cannon or flamer. slow)Conqueror
The Conqueror is very effective against infantry, and its broad range of weapons suit it for every situation from support to close assault. 5 hard points (armed with auto-cannon replacing right arm which can be substituted with a boltgun siege hammer power fist, heavy bolter on backwhich can be replaced with a las-cannon or meltagun, and power fist in left arm which can be substituted with an auto-cannon flamer or seige hammer)
It is accepted practise amoung the forces of the Imperium to give each Robot a three-part code after the class name, to indicate its weapon configuration.
The first element of the code refers to the back-mounted armament, the second element to the right arm and the third element to the left arm. Letters are as follows.
Heavy Bolter B
power fist g
Power sword s
No weapon x
Upper-case letters are used for heavy weapons, and lower case for normal and close combat weapons. Thus a Colossus class robot like the one shown, equiped with a back-mounted melta-gun, a bolter on a right arm and a seige hammer in place of the left, is referred to as a Colossus mbH.
Robots need 1 Hard Point (HP) for each basic or close combat weapon carried, and 2 HP for each heavy weapon. These 2 HP for heavy weapons are, however, combined into a single mounting for the weapon. The seige hammer requires 2 HP when fitted to a robot.
all info taken from Codex Titanicus, with Robot Information added from Warhammer 40,000 Compendium