Back at home, when he has time to examine it more closely, it quickly becomes obvious why this ritual never turned up in all his early research. It's not a typical soul summoning ritual. It's not a cultivation ritual at all; as Nie Huaisang puzzles through it, the closest category he can put it in is folk magic...not only that, but sex magic.

Wei Wuxian doesn’t let the Wen Remnants hand themselves over after Jin Zixuan’s death. Instead he makes one last sacrifice to ensure they survive. Months later, a Lan cultivator kneels at the edge of the Burial Mounds, ready to offer himself to the feared Yiling Patriarch. Or: Sacrifice is an old magic, but love is even older.