Soullessness

Being soulless has a variety of effects on creatures, most of which are inconvenient. The most obvious detriment is that memories do not get backed up to a soul if the soul isn't connected to the body, and therefore to the brain, and therefore to the mind. The most impactful detriment is the loss of the creature's innate resistence to ambient magic.

Causes

To render a creature soulless, all of its souls must be disconnected from its body. This generally counts as difficult, and invariably requires magic of some sort.
Humanoids are the least difficult type of creature to strike soulless, since they default to having only one soul and it is external instead of "within" the body. Most humanoids become soulless if anything severs the spirit that connects their soul and body, which is easier than digging out an internal soul but is also still easier said than done.
Celestials are often capable of soul management, allowing them to cause soullessness if they find cause to do so. Fae deals often find their way to being enforced beyond the skill of the enforcer, so promising souls to fae usually works. Fiends are capable of dealing damage on a variety of levels.
Drastic transmutation of the body can weaken its connection to the soul, possibly breaking the connection entirely. This is especially true if multiple changes of the same magnitude are made in quick succession. Some workarounds alleviate the risk to an extent, but it remains a potential problem.
Last but not least, most creatures are rendered soulless when they die. This is the least problematic time of onset; in fact, it's generally less of a concern than the soul remaining in the corpse. Still, the departure of the soul is the reason that ordinary healing magic stops short of raising the dead.
A few unique cases could expand this list beyond what it already includes, but are not common enough to warrant inclusion.

Symptoms

Without the defense against ambient magic, creatures' bodies can be warped by their environment, or by a strong enough internal push. The especially resourceful can turn this to their benefit, self-shaping into a new sort of creature of their own design, but it requires a lot of willpower and a lot of luck.
Without an easy source of personal power, magic becomes incredibly difficult. Even the most basic defensive magic requires a power source. Injuries land more easily and stick around longer when the body can't deflect and heal them with soul energy.

Treatment

The loss of a soul is technically reversible — most commonly in the form of resurrection, since it's easier to secure the patient's cooperation in those cases.

Prognosis

Arguably, this counts as being dead. Loose souls find their way to an afterlife unless obstructed.

Prevention

As a mostly inflicted condition, soullessness is hard to guard against in the same way that murder or having one's arm bitten off is hard to guard against. It can be made more difficult for attackers, but not every factor is going to be within the victim's control.
Some techniques exist for strengthening the connection between body and soul. Meditation is the simplest example.

History

The first large-scale onset of soullessness, and the largest-scale events to date involving the condition, were the Soul Crises in Eastern Espere and Southern Espere. They were enacted by Dubain and Ameya, respectively, as part of the Patrons' feud. Initially assumed to be one event, the Soul Crisis contributed to era-ending wars across the continent.
In the Southern Esperean Soul Crisis, a single celestial agent created by Ameya was responsible for striking the region in a series of random attacks. This was resolved when a convincing argument turned the attacks in a more pointed direction, and the attacks ended when Dubain was deposed.
The other Soul Crisis was not resolved as cleanly. Rather than being severed outright, spirits in Eastern Espere were folded, bound, and set ablaze like candles. While information and energy could still cross the gap so long as the "fire" was lit, these candle-spirits were much more fragile. The connection burned itself out over a long enough timeframe, and extinguishing the "flame" was easier than cutting through an intact spirit. Even a natural exchange of mental or magical energy could be enough to "blow" the candle out. Because of the delayed onset from this method, even without additional spirits being candle-bound, the Soul Crisis was assured to be drawn out long after Dubain and Ameya withdrew from Espere.

Type

Magical

Cycle

Chronic, Acquired

Rarity

Uncommon