The Sword Demon Patriarch fled, but Lord Dog did not stop him. It was not easy to stop a Nine-revolution Great Saint from leaving, for such an expert was already the supreme existence in this world. To achieve that, one would have to pay a great price.
Lord Dog could do it, though, but he did not want to expose his trump card for a mere Sword Demon Patriarch.
What caused the Patriarch to leave was the death of Black Demon. The fall of a Nine-revolution Great Saint was by no means a minor incident. It could shake the entire Netherworld, especially since Black Demon had a unique status. He was the Lord of a forbidden land in Earth Prison, and his death meant the fall of that forbidden land.