"The Jade Pendant!"
Gu Fan remembered suddenly.
In the original plot, after Lin Shu died from her chronic illness, the servants had callously thrown her meagre belongings into the trash heap behind the kitchens.
And as 'luck' would have it, Gu Chen, wandering the grounds in a rare trance of melancholy (farming aura points among the female disciples), found the trash, and among them the jade pendant covered in the mud.
Instead of ignoring it like anyone else would do, he picked it up and cleaned it, only to discover that it was a sealed map, and used it to find the Blue Ghost Fire.
That fire was the absolute cornerstone of Gu Chen's rise to power in the early arcs. It was the ultimate weapon for a poison cultivator.
Blue Ghost Fire was a ruthless offensive weapon, a strong defensive wall, and a very potent means to concoct high quality poisons as well as potions.
