Wars of Secession
The Wars of Secession were a series of civil wars that wracked the old Necrontyr Empire.
The First Wars of Secession
Because the Necrontyr were so short-lived and fixated on merely surviving until death, their loyalty to Dynasties other than their own was not stable. Many Dynasties chafed under the rule of the Triarch and sought to break away. To restore the Empire's unity, the Triarch designated the Old Ones as common enemies of the entire Empire for not giving up the secrets of eternal life, initiating the first War in Heaven.
The Second Wars of Secession
The War in Heaven didn't go so well for the Necrontyr, because the Old Ones were just that powerful. Furious with their leaders' bungling, even more Phaerons attempted to break away than before. Had the Old Ones put their minds to it, they could have wiped out the Necrontyr on the spot (and saved the galaxy a bunch of trouble later), but they didn't. Instead, the C'tan made contact with the Silent King, who used them to end the Wars of Secession permanently by stripping everyone of their free will.
After this, the newly-minted Necrons went another round with the Old Ones and beat them, and then betrayed, shattered, and bound the C'tan, but left the galaxy so messed up that the Silent King put everyone to sleep for sixty million years to let it heal up. Before he left into self-imposed exile, he returned the free wills of his people (the ones who had enough minds left to have free wills, anyway, which is pretty much just the nobles and the scientists while everyone else got stuck as mumbling zombies at best and trapped in their bodies at worst), leaving only one last command: to restore the Necron Empire to galactic domination. He didn't name any successors, and he didn't leave any particulars about how this was to happen, so now that Tomb Worlds are reactivating all over the galaxy, the possibility for a third phase of the Wars of Secession is open...
Making things worse is that a few Necrontyr managed to escape becoming automatons and have returned to waged war against their undead brethern.