I called in my fairy familiar after finishing the golem. Naturally, she exited her body and entered the golem's body. The body she was possessing disintegrated, and suddenly, the golem's body started glowing. It seemed the golem was good enough for her physical incarnation.

A spirit possesses a body in order to recover mana. When they resist the energy of the material realm, it places a great burden on their soul. The energies of the spirit world are much less volatile and cannot be blocked by physical matter, unlike in the physical realm. Spirits, being energy-based life forms, do not usually possess a physical body. Only spirits under a long-term familiar contract will bother possessing something like a body or an object.

This is because the person they have a contract with supplies them with mana to maintain their presence in the physical realm. The amount of mana they have is not indefinite, so possessing a body allows them to take a break. They can conserve their mana by having a physical body, as it's not constantly being drained by resisting the energies of the material realm.

Suddenly, the entire galaxy began shaking violently. It was an instinctual understanding that the entire galaxy was indeed shaking.

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The sky began filling with golden clouds as majestic and holy golden rays started illuminating the surrounding area. Numerous drums could be heard, though it was unclear where the drumming came from. The sound echoed: boom, boom, boom, boom. The drumming grew louder and louder, and suddenly a pillar of light descended on the body.

"Oh, it isn't just a physical incarnation," I thought. "It looks like she's evolving into a celestial fairy."

There are many conditions physical incarnation be one of them. It seems I was right about my suspicions that she wasn't an ordinary spirit but a very powerful one. Finally, after an hour, her physical incarnation and evolution were complete.

Unlike possessing a body, a physical incarnation involves the spirit gaining a physical form. This requires that the body be strong enough to allow them to do so. Even weaker spirits would still have relative trouble finding a strong enough body for their physical incarnation. The problem for the weakest spirits is less about finding a suitable body and more about having enough power to create a physical incarnation when they do find a suitable body. As they become more powerful, the body needs to be stronger, creating a vicious cycle where they either have insufficient power or they don't have a sturdy enough body.