Fateful Night in Togenkyo (1) - S1.E57

After Sango leaves to repair her damaged Hiraikotsu, Inuyasha and the others come across a demon tree and a sage known as Tokajin, who uses humans to feed the tree so it can grow a fruit of immortality. Not only do the four manage to be caught by Tokajin in some way, but it's also the night of the new moon, when Inuyasha becomes human.