Complex sentence: However, unbeknownst to the user, the ritual also brings the subject into contact with the Hydra, which then merges with the individual's astral self, using it as a host.
Split into simpler sentences: However, unbeknownst to the user, the ritual also brings the subject into contact with the Hydra. The Hydra then merges with the subject's astral self, using it as a host.