The Ultramarines are a prominent chapter of Space Marines in the Warhammer 40,000 universe, known for their strict adherence to the Codex Astartes, a set of guidelines for Space Marine conduct and organization. They are recognized for their blue armor and are often seen as the epitome of honor and discipline among the Imperium's forces. Founded during the Great Crusade, the Ultramarines are based on the planet Macragge and are led by their Primarch, Roboute Guilliman. They play a crucial role in defending humanity against various threats, including Chaos, Orks, and Tyranids, making them one of the most respected and effective military forces in the galaxy.