Death Mushroom, an extraordinary poison!
The Notionless Saint was a Small Saint in any case, yet he had just stretched out his Origin Power into the area where the Death Mushroom was growing, and had not even touched that Saintly medicine, and he had already sustained such a terrifying backlash.
If he had really used his hand to make direct contact, what would've happened?
His body would've probably directly rotted into a pool of mush.
This Death Mushroom was comparable to a Saint King in itself, the only difference being that it would not make any active movements. A Small Saint was naturally delusional if he wanted to harvest it.
The Notionless Saint quickly drew out some alchemical pills, ingesting them. The damage that came from severing his own arm was still quite large; though a new arm immediately reformed and regrew, it had still used some of his life force. This would naturally have damaged his foundations.