A Moon Spirit has no physical form; although many wizards have speculated that it doesn't have an astral form either. This is based off the fact that several spells designed specifically to ensnare and detect astral forms have no effect on moon spirits. Instead, many wizards agree that the moon spirit is some type of inverse shadow creature. Like the natural world creates the shadow realms, so does the moon create its own shadow realms. Normally these realms never touch, but during the time of the full moon the two realms make brief contact, which can result in a moon spirit being pulled out of its realm and cast into this world. These same wizards speculate that the moon spirits are either natural shape shifters or parasites in their own realm.

Moon spirits seem only capable of possessing a creature under the light of the full moon. Nobody knows why this is, but some have speculated that the moon spirits draw more strength from their realm during this period. It will stay in possession of this host until it has grown strong enough to leave and find another. After each possession the moon spirit grows in strength and power - with the host gaining from the strength of the spirit. It is quite possible for a seal possessed by a powerful enough moon spirit to no just drive off a polar bear, but actually kill and devour it. Their doesn't appear to be any upper limit to a moon spirit's power and northern legends speak of a single polar bear slaying two giants and feasting upon their entire bodies.

Moon spirits will only possess carnivores or omnivores, which has led to speculation that the moon spirit needs animal matter to survive. In fact, the host will only eat living prey, meaning that many scavengers that get possessed will stop eating carrion and favor fresh prey. However, the spirit does not seem to care if it kills its victims or steals fresh prey from some other creature - as long as its fresh. Natural creatures will always avoid creatures that have been possessed by a moon spirit. This goes so far that mothers will abandon their children if they become possessed.

Once a moon spirit has been forced to leave its host, or does so on its own accord, the host will be sleepy for the next few days but most will survive the experience. Within a week the ex-host will return to its normal pattern, but it will still be shunned by normal animals and any human with the “sight” will sense something unnatural about the ex-host.</p>

They have no natural physical form; although when they possess a physical body they maintain that body's form. There are a few minor physical differences between the host's natural form and the host after it has been possessed. First, the hosts eyes will become milky white orbs when exposed to moonlight. Second, the host will become ravishingly hungry. This hunger will continue to grow the longer the moon spirit possesses the host. What consumes this food is unknown, because while possessed the host creature will actually lose weight. Third, the moon spirit possessed host will avoid being outside during the daylight hours. It seems uncomfortable during the dawn or dusk, but will move during this time if it must. Finally, when the host is under a great emotion or feeding they will give off a slight whitish glow. This glow is impossible to see in the daylight, but is perceptible in total darkness.

Before a moon spirit finds a new host, or is forced out of their host, they resemble a small wavering cloud of moonlight. The moon spirit is very vulnerable in this form and will seek to find a new host as soon as possible. This cloud of light can move in small bursts of very fast speed, bursting for a few feet then coasting for a dozen more. If exposed to sunlight while in this form the cloud will disappear, which most people believe means the creature has been destroyed.

Its natural habitat is unknown, but speculated to be one of the moon's shadow realms that is very close to the moon itself. On this world moon spirits have been found in just about every environment; although they seem to be more common in the far north where the sun's cycle is longer. Even while possessed the host must still live in whatever environment it normally lives in; however, it is far hardier than other of its species.

When in a host someone is mostly likely to encounter a moon spirit that is very hungry. It is possible the host will not be hungry, but this will only be after it has just eaten a large meal and it is still early in its birthing cycle. Otherwise, if the host could eat them it will attack, kill, and eat anything it encounters. The moon spirit will attack with whatever weapons the host has at its disposable. If the host's natural weapons are not up to the task the moon spirit will use its own life force to give the host the power it needs to kill. This power seems to be able to do just about anything the host needs to kill its target.