Water or Blood
A figure called The Darkling Prophet started a mystery cult focused on recruiting lesser members of The Blood, giving them promises of power and a network that made some of those dreams a reality. The prophet preached the unfaithfulness of the blood to the gods and that a new order must rise to show them proper respect. He said that the blood of the gods had grown thin and the time of the great houses was past.
The movement build strength in relative peace for years, until the centennial massacre where many great lords and high blood where slain at their celebrations. Many of the legions “revealed themselves under new rule” and where said to be following the words of the prophet. Unfortunately for the movement The God’s Watch saved the imperial family and a few members of the other great lines.
The Emperor infuriated by this ordered a purge of all Thin-Blood rebels. Placing his God’s Watch in charge of the extermination. His armies marshaled to his side and in a bloody campaign, where huge massacres of rebels were a routine thing, he crushed army after army, purging (sacking) his own cities as he reconquered them. He became called the weak for his inability to control his own people.
It took twelve years, but eventually the prophet’s forces were annihilated, But the emperor never lived to see that as he died to rebels in his own household in the final year of the revolt. The prophet vanished before he could be captured, and has occasionally been spotted both in visions and in mysterious books that have cropped up ever since, it is said one day he will return.