Complex sentence: How an original dragon from Thanatos succeeds in obtaining a Resonance with a human is unknown, but it appears as though they can achieve it without the human's consent, and the human may not even be aware that they have completed a Resonance with a dragon.
Split into simpler sentences: How an original dragon from Thanatos succeeds in obtaining a Resonance with a human is unknown, but it appears as though they can achieve it without the human's consent. In some cases, the human may not even be aware that they have completed a Resonance with a dragon.