The Nightmarian Controversy was the debate among the proto-Overseen religious movement that began forming shortly after the Catastrophe. At issue was the question of whether Princess Luna and Nightmare Moon were two separate individuals, with Nightmare Moon having kidnapped and imprisoned Luna, or were the one in the same, with Luna herself being identified as Nightmare Moon.
In the first years, the identity of Luna with Nightmare Moon was the more popular position, largely accepted based on historical texts. This began to change after Night Writer wrote an influential treatise on the nature of the Princesses in which she advocated for the strictly good divinity of the Princesses and denounced the belief that a good princess, thus also a goddess, could be corrupted to become a nightmare. The counter-argument of the Nightmarists, as those who held to identification of Luna with Nightmare Moon came to be called, was that their position was better supported by well-regarded historical texts. They also claimed it was vital to a deeper understanding of the nature of the Princesses that they possess the same capacity for darkness as exists in the nature of mortal ponies, and that attempting to separate out the good from the bad in them by blaming another entity would make the Princesses essentially unknowable aliens, rather than exemplars of ideal ponies who succeeded in meaningful struggles to choose good over evil.
Eventually the issue was decided at an ecumenical counsel, in favor of the position that Luna and Nightmare Moon were separate beings. "Nightmarism" was nominally declared heretical, though for political reasons it was rarely actively sanctioned and remained persistent enough to never truly die out.