The Black Templars are a Loyalist Second Founding Space Marine Chapter derived from the Imperial Fists and their Primarch, Rogal Dorn. Their origins can be traced back to the Imperial Fists' defense of Terra during the Horus Heresy.
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Since then, the Black Templars have been on the longest Imperial Crusade the Imperium of Man has ever experienced, to prove their loyalty to the Emperor of Mankind. They are not a Codex Astartes-compliant Chapter and maintain a very different chapter structure and culture than is the norm among most of the Adeptus Astartes.
They are also highly unusual among the Adeptus Astartes in honoring the Master of Mankind as a god, just like the mortal followers of the Imperial Cult do.
After the Horus Heresy, Rogal Dorn refused to break the Space Marines into smaller chapters, it was only when the Imperial Fists were nearly branded as heretics that he relented, allowing his beloved legion to be divided. One of these new Chapters was the Black Templars.
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