The Imperial Knights are noble houses that operate the various Knight Battlesuits. Smaller than Titans, but significantly larger than most vehicles, and being more numerous and easier to maintain, the Knights are a potent force-multiplier in any conflict. Many are sworn to the Adeptus Mechanicus to watch over feudal worlds that keep the Mechanicus' Forge Worlds fed, though they will also be called upon to assist in campaigns against entrenched enemies.
An Imperial Knight that falls to Chaos will become a Daemon Engine, and loyalist Imperial Knights have sworn to hunt down all such traitors.