Another piece of evidence of that martial demonic aura is just the thing to corroborate this.
And Tan Shuchang was preoccupied with this martial demon because when it first emerged, it was dead. A single touch from the Sword Spirit of the broken Immortal Sword allowed him to wield his fierce ghostly power once, and his overall strength underwent a quality transformation akin to shedding an old self. Slaying those elders from the Sword Mountain was as easy as slicing through vegetables.
Therefore, Tan Shuchang felt that if he could "devour" a martial demon, his strength might increase even more significantly.
After all, something with such a notorious reputation in the Jiawu Realm couldn't possibly be as superficial and weak as the Sun-Walking Spirit Envoy he had previously refined from the forbidden practices of Sichong Gate.
Soon after, Tan Shuchang left Black Heart Mountain.