Unrest on Caliban
The unrest began shortly after the campaign against the Sarosh that saw Luther and many other Dark Angels quasi-exiled to their homeworld by Lion El'Jonson. It quickly became apparent to Luther, Zahariel, and the others that Caliban was plagued by a resurgence of the Warp-infected Great Beasts and rebels of disgruntled Calibanite nobility and a mysterious cabal of Terran Sorcerers that had infiltrated the world through the local Administratum outposts.
After terminating the rebels and their Sorcerers, Luther realized that the Warp-taint on Caliban was not only the fault of the Great Crusade which had brought the treacherous Terrans in the first place, but that should the Emperor discover the planet's malignancy Caliban itself could face Exterminatus. Thus Luther and his followers declared independence from the Imperium, setting the stage for the Destruction of Caliban at the end of the Horus Heresy.