Let's talk about Daoist Equal to Heaven, who upon receiving the news of enlightenment, learned that this evil being was an arm severed by Hongjun. They didn't know to whom it belonged, but it was frightening enough. An Ancient Immortal's formation couldn't keep it sealed. Just the arm alone was enough to send shivers down one's spine.
Mount Shu has been a place of exceptional people and spiritual land since ancient times, with many high Daoists emerging from it. It's unclear who these Ancient Immortals were, but they must have been peers of Rain Mage Chi Songzi and Feng Bo Fei Lian, or else they wouldn't have been able to seal this arm for so many years.
Sitting at Guan Jiangkou and discussing the Dao over wine, Daoist Equal to Heaven said to Lu Dongbin, "Pure Yang Daoist Friend, I have a matter of great importance to discuss with you, something that concerns the safety of the Shushan Sword Sect. I dared not delay."