Back then, the Demon Ancestor attempted to instigate chaos among the three clans. He corrupted many top-notch experts from these clans with his secret demonic method. Of these unfortunate souls, this old dragon was one of them.
Once corrupted by the Demon Ancestor, one essentially falls into demonic path and becomes enslaved by the Demon Ancestor. Any chance of salvation is nearly nonexistent. This old dragon most likely was detected by the Ancestral Dragon, leading to this disaster.
However, it also revealed the tremendous strength of Ancestral Dragon from the catastrophic fate of this old dragon. A prestigious Dao Monarch was battered to death with just a single punch from the Ancestral Dragon—an astonishing fact beyond Jiang Chen's imagination.
Although the Dao Monarch is deceased, his might still commands respect, a power Jiang Chen currently couldn't afford to disrespect. Thus, he didn't touch the dragon corpse, instead focusing on exploring the grand hall.