The Bellis Rebellion
From Warhammer 40k - Lexicanum
In 913.M36 the gem mines on the moon of the planet Bellis, proved so lucrative that their masters came to believe themselves beyond the rule of Terra.
Having built up formidable defenses and equipped their private armies with the finest arms and armour they could obtain, the lords of Bellis declared their secession from the Imperium. The Imperium's response was a year in coming, but when it was finally delivered it took the form of a Battle Company of the Dark Angels Chapter. The Dark Angels descended to the surface of Bellis in force and crushed the rebels; the masters of Bellis were rewarded for their hubris by being subjected to the less than tender mercies of the Company's Interrogator-Chaplains.