The last time Hollow Ichigo was seen in the story was just before he merged with Tensa Zangetsu in Ichigo's inner world. It has been revealed that due to the Hollowfication infection passed on to Ichigo by Masaki Kurosaki, the Hollow took the place of his Zanpakutō and this make Hollow Ichigo his actual Zanpakutō, Zangetsu. The impostor is actually a representation of Ichigo's Quincy Powers which is why he resembles a youthful Yhwach.