Celestials

"The sun sets behind the mountains,
And the Yellow River flows into the sea.
To thoroughly enjoy a thousand-mile sight,
Climb up another level.”

— Wang Zhihuan, "Climbing Stork Tower"

Celestials are the servants of the gods, living inside the realms of the Spirit World. They are their own society with their own culture, history and customs, made up entirely of immortal spirits.

Creation: Celestials are created from the souls of the deceased that enter and become inhabitants of a Deity's domain.
Usually this is because they worshiped the deity in life, because they were captured from another god as spoils of war or because they found the domain by chance and were allowed inside. After just a few short decades, they become natives of the domain and thus proper celestials.
A celestial's powers are strongly influenced by the owner of the domain they inhabit, as well as their own personality.

Divine servants: Celestials are the servants of the gods. They live and work much like they did in life, though usually with much less hardship. They serve as workers, soldiers, champions and more, in support of the owner of their domain and protected in turn.
Most celestials want nothing more than to spend eternity enjoying their deserved afterlife in relative peace and quiet. Yet others are active, engaging in courtly affairs and improving their martial prowess, aiming to climb ever higher.

Mortals: celestials largely do not care about the mortal world at all, unless it is their job to do so.

Ranks

Celestials come in different tiers of power and influence, gained through the centuries they spend in the spirit world.

Servants
Most celestials are servants, menial servants, attendants and the like. They while away in quiet peace, working hard and enjoying their free time as they please. They are not especially powerful and do not usually venture out of their patron's domain.

Envoys
Envoys are more powerful celestials, either through being granted more power from their patron or through self-improvement. The armies of the gods are largely made up of envoys, and as the title suggests they are often messengers between domains.

Heralds
Heralds are champions, representatives of the gods in battle and elsewhere. These beings are very powerful in their own right and hold great sway in the court of their patron.

Exiles
Occasionally there exist rogue celestials that wander the spirit wilds unbound. They were either exiled, broke away from their patron or survived when their patron was destroyed and have not joined the court of another divine since.
Without the protection of a deity, survival can be difficult, so rogue celestials are usually fairly powerful.

Appearance

Celestials are spirits and as such don't have a fixed form. When they are summoned they usually take a shape that fits the perception of the summoner.