Phantom
Phantoms are immaterial ghosts with no form of their own, who exist only within other objects or creatures. To ask what a phantom looks like on its own is a meaningless question; they are more forces than objects, and cannot exist outside a host. A phantom's host, however, can be nearly anything, which gives phantoms (as a group, if not necessarily individually) enormous versatility.
Possession
When a phantom first divests, it is already in possession of an object, generally something on his person when he died, or at least something very close by. Some phantoms simply remain in their original hosts, but most phantoms seek out a more useful and suitable form; they seem to instinctively know, on divestment, that they have the power to possess other objects.
To possess an object, a phantom must generally be touching it—that is, touching the new object with the object the phantom currently possesses. Some phantoms, however, develop the ability to possess objects at a distance, apparently instantaneously translocating somehow from one host to another. Even then, however, there are limitations to the distance; the phantoms that have developed the most ability in this regard still only seem to be able to possess objects no more than five meters away at best, and for most the possible distance is much shorter. In any case, once contact is established or the prospective new host is in range, possession is an act of will that happens more or less instantly; the phantom simply leaves possession of its old host and takes possession of the new one. The process is somewhat fatiguing, however, so a phantom generally won't do it too often; most phantoms stay with a host for an average of at least a day or so, though there is wide variation in individual practices.
Types of host
A phantom can possess virtually any object, though there are some limits as to the mass. However, they do tend to prefer hosts of generally humanoid form. Statues, mannequins, dolls, suits of armor; even snowmen and scarecrows, make fairly popular hosts. Some phantoms prefer animal form, possessing stuffed animals (either real animals stuffed through taxidermy, or stuffed animal toys) or animal-shaped statues and other objects. Not all phantoms choose such hosts, however; some simply possess handy objects with no appearance of life. Among the most common hosts not of human or animal form are clothing, sheets, and various kinds of electrical appliance.
The limits of a phantom's possession seem to vary slightly by individual. Some phantoms have been known to possess objects massing as much as a dozen metric tons, or as little as a fraction of a gram, but for most phantoms the limit seems to be about six hundred kilograms on the large end, or fifty grams on the small end. It is possible for a phantom to possess only part of a larger object (a head or arm of a huge statue, or a particular section of a complex machine), but that does not convey the phantom any power to detach the possessed part from the whole, or to move the entire object about, so such a phantom would be severely limited in mobility. Some phantoms develop the power to possess collections of similar small objects (grains of sand, marbles, haystacks) or even bodies of liquid or gas, though this entails some danger; if the objects become separated, or the liquid or gas mixed into a larger body, the ghost may have a very difficult time collecting itself back together.
The phantom is not entirely limited to the host's usual abilities, particularly with regards to motion; a phantom can cause a possessed object to move and deform in ways normally impossible. A phantom possessing a statue, for instance, could cause it to move its limbs and articulate more or less like a living person, albeit with less fine control. A phantom possessing a marble bust, however, would have much more restricted mobility than one possessing a full statue with arms and legs; the bust would be able to twist and bend to some degree, but without limbs would still be limited in its movements. Even aside from mobility, the host, naturally, determines to a great degree the phantom's physical properties. A granite statue would be able to take more damage than a plastic mannequin, and a paper doll could be damaged on getting wet whereas a rag doll may not.
Living hosts
It's possible for a phantom to possess a living host, though it's substantially more difficult, at least to maintain the possession for any length of time. Living creatures have their own souls which typically object to the intrusive presence of the ghost and fight the possession. In the case of alogous creatures, this opposition happens subconsciously, but it still takes place. The difficulty of establishing and maintaining possession of a living host depends to a large degree on the mind and willpower of the host in question. Unintelligent creatures such as insects and worms may be easy to possess, and easy to retain possession of indefinitely, but are generally considered less desireable as hosts. Humans, conversely, are able to consciously fight the possession, and are much more difficult to control. Nevertheless, some phantoms are able to overcome a human host's resistance and keep possession of the body, but it's a constant struggle to do so, and the host generally manages occasionally to briefly reassert itself before the phantom takes control again. Eventually, the host is generally able to gather the will to expel the phantom from its body; some phantoms are able to hold onto possession of weak hosts indefinitely, but more commonly possession of a human host lasts a few days at most, and more often only a matter of minutes while the phantom takes advantage of the temporary body to wreak havoc in the Mortal World.
Ghostly hosts
Even other ghosts are subject to possession by phantoms—phantoms cannot possess other phantoms, since they have no physical or even ethereal form to possess, but they can possess any of the other major types of ghost; shades, specters, apparitions, wraiths, and ghouls are all susceptible to phantom possession. While in possession of another ghost, the phantom for all practical purposes becomes that ghost, with the full potential to use all its powers and abilities. The original personality of the ghost it is possessing is supplanted by the phantom, not truly annihilated—after all, it will come back to the fore when the possession ends—, but suppressed during the possession.
Ghostly possession seldom lasts long. Ghosts can throw off possession even more readily than mortals, and a phantom has rarely been known to stay in possession of another ghost for longer than a few hours.
Residence
Many phantoms have chosen hosts in which to take permanent possession. Such a permanent host is known as a residence. Establishing a residence requires remaining in the host for a long period of time, during which the phantom focuses on joining itself with the host and claiming it for its own. The exact period of time required varies, but is at least a week. The phantom need not be consciously thinking of creating a residence, or even have any intellectual idea that such a thing is possible; there have been many cases of phantoms establishing residences apparently only by wanting to forge such a permanent possession, and subconsciously exerting the will to do so.
Once the phantom has established a residence, it cannot then leave it to possess other objects; effectively, the residence becomes the phantom's body. If the residence is destroyed, the phantom itself is usually destroyed as well, though rarely some phantoms manage to make the leap to another host at the moment of their residence's destruction. Balancing this restriction, however, the phantom does gain a number of benefits from its residency. Damage to the phantom's residence heals at a rate similar to that of wounds on a living body (though some phantoms develop the ability to heal their residences more quickly), and phantoms tend to have finer control over the motions of residences than over other hosts. Frequently, residences will alter their forms slightly to resemble the phantom's original body; a featureless mannequin that becomes the residence of a phantom, for instance, might over time develop the facial features of the person the phantom was before he died.
Living residences are possible, but far from common. Even though some phantoms have managed to retain possession of living hosts for extended periods of time, the continual effort involved usually prevents the establishment of residency. Most of those phantoms that have established living residences reside in small, unintelligent invertebrates and fish; only a few phantoms have achieved residency in mammals, and human residences probably number in the single digits. Ghostly residences are even more unlikely, given the greater ease of ghosts to fight possession. It may be that the only practical way for a phantom to permanently reside in another ghost is for the host to willingly submit to the permanent possession. While this has actually happened in at least one case—that of R. Luca Veit and Trent David, who later became part of the eidolon known as Styrle—, it's extremely rare at best. In tongue-in-cheek reference to its best (and in fact only) known "victim", this process of permanent possession is sometimes called "arlucavation".
Phantom abilities
Some phantoms may develop a number of unique abilities unknown to other types of ghost. Some of these abilities have already been mentioned, such as the power to heal damage to their hosts, or to possess new hosts at a distance, or to possess liquid or gaseous hosts, or multiple small objects. Other abilities some phantoms develop include levitation, the ability to produce various sounds by vibrations of their hosts, and the ability to make their hosts partially or fully invisible. Some phantoms can effect limited transformations of their hosts—usually altering their shapes in minor ways, though a few phantoms can actually change their hosts' substance. A few phantoms who become particularly skilled in possessing living hosts may possess some ability to control living creatures at a distance, even those they have not possessed.
Unlike most ghosts, some phantoms are able to pass outside of haunts in the Mortal World without entering the Gap. Like ghouls, these phantoms, when they leave the boundaries of a haunt, simply pass elsewhere in the Mortal World like ordinary mortals. Other phantoms, however, do end up in the Gap on leaving haunts, like most other ghosts. Such phantoms bring their hosts with them when they pass into the Gap, one of the few ways that material objects can enter it. Even if carried by a mortal or otherwise brought outside a haunt by outside means, these phantoms—and their hosts—will pass into the Gap when they cross the haunt's boundary. This may produce surprising results for a nearby mortal, to whom it may appear that an object he was carrying simply disappeared for no apparent reason. What determines whether a given phantom will enter the Gap on leaving a haunt or will remain in the Mortal World isn't yet understood, but it generally works consistently for a particular individual. Some phantoms develop the ability to choose whether they pass into the Gap or not when they leave a haunt, and a very few seem to randomly pass into the Gap sometimes and remain in the Mortal World other times, but these are exceptions to the rule.
Regardless of whether they can enter the Mortal World or the Gap or both, however, all phantoms can still pass between the Mortal World and the Spirit World through mirrors, like any other ghost. However, unlike most varieties of ghost, phantoms cannot pass through just any mirror; the mirror must be large enough for the host to fit through—although they may reach parts of their hosts through mirrors too small to admit the entire host. A phantom in possession of a life-sized statue, for instance, could only enter the Spirit World through a mirror large enough for a human to enter its frame. It might be able to climb and crawl through a mirror on a medicine cabinet, but it would be a tight fit (and might break the cabinet while the phantom holds onto the frame). A typical car rear-view mirror would be far too small for the phantom to get through (unless it possesses a smaller host, at least), though it could possibly reach an arm through to grab something in the Spirit World on the other side.