How did Unalaq learn to do spirit-calming? Did he learn it from the Spirit Owl who guards the Library in the spirit world?
None of the source material (shows, comics, etc) ever specifies. But here is what we do know:
- Unalaq joined and was taught by the Red Lotus and its members, but this was after he demonstrated his abilities to calm spirits. He did not learn it from them, and it is likely he taught the Red Lotus about Wan and the Avatar Cycle after learning it from spirits.
- Unalaq can heal humans with waterbending. If spirit anger is considered a 'malady', it could be a related skill.
- Very few individuals interacted with spirits at all before Harmonic Convergence. It is possible his spirit calming abilities are less unique than his ability to contact spirits, and other waterbenders could have mastered the technique if they had any reason to practice it in the first place.
- Jinora was able to enter the Spirit World and communicate with spirits prior to Harmonic Convergence. It is likely Unalaq simply has the same natural talent.
- The Spirit Oasis is situated at the Northern Water Tribe capital. As the son of the chief, Unalaq would have had access to this spiritual center.
If I were to guess, I'd say Unalaq had the same natural talent Jinora has, and that he used the Spirit Oasis to help him reach the Spirit World. Once there he could have learned a great deal from any spirit there or simply taught himself the technique.