The Seven Noxious Divine Yin Pill itself was not poison. It was an extremely oppressive frost-based supplement. If one was cultivating in extreme frost cultivation techniques, and one's cultivation level was high enough, one could resist the frost toxins in the Seven Noxious Divine Yin Pill, then it would only be beneficial to consume the Seven Noxious Divine Yin Pill.
However, for Yi Yun, who was lacking in cultivation level, and a injured Shentu Nantian, eating the Seven Noxious Divine Yin Pill was equivalent to eating a deadly poison.
Back then, Yi Yun had used the Purple Crystal to control the Seven Noxious Divine Yin Pill's frost toxins and sealed it all within the Purple Crystal. It was ranked alongside the energy essence, the Pure Yang spirit, that he had obtained at Fallen Star Gate.