The Dark Qilin was like a doomsday monster. For the Nether Prison experts, it was simply a disaster when such a terrifying quasi-divine-beast joined the battle. None of them could stand up against its attack.
A Qilin was an auspicious sign and a divine beast. However, a Dark Qilin was a mutation of it. A quasi-divine-beast that represented death and slaughter, it lived in the Abyss and was an iconic expert of the place, just like Abyssal Demons.
When the Nether Prison invaded the Abyss, it wantonly massacred Abyssal Demons, and its supreme Great Saints even banished the Dark Qilin. Without the protection of the Dark Qilin and Abyssal Demons, the Abyss was like a turtle without its shell, unable to resist the Nether Prison's invasion. In the end, the whole of Abyss became a vassal of the Nether Prison.