The Masks are magical prisons created by the Magi to trap the various supernatural, non-human entities of the world. Though the knowledge of the creators is currently lost, they were carefully designed by the mascherari, each created specifically for the spirit they were meant to contain. They are are magical spaces, almost realms of their own, which the spirits are locked in to prevent them from interacting with the physical world. However, this does not stop them from interacting with magical practitioners who make physical contact with the Mask, called hosts or owners. More often than not, this results in the spirit possessing the mortal (free movement in the physical world in exchange for power) until the power imbalance destroys the body or the mortal is caught and killed. Either way, the result is the same: the spirit is trapped in the Mask once again until it can find another host.
Most of the Masks created between the 700s and the 1600s, during the height of the mascherari numbers, are unique and robust; as the Magi were expelled from most of the Old World in later years, surrogates and generic masks were used for quick containment, and then never properly replaced. While a specially crafted Mask is almost always created for the incredibly powerful spirits, the more mundane are often trapped in basic masks or even other objects. This can lead to a quick corruption of the entity, eroding their memories and diluting their powers, or introducing mutations to their abilities. A ruined spirit can cause untold destruction and decimates the mind and body of its host at a much faster rate. A specially crafted Mask preserves the spirit for centuries or even millennia, though the isolation also has many deteriorating effects.
Trapping a spirit will influence the world, particularly regions where the entity was highly regarded. The imprisonment of deities, like the imprisonment of Kali in her Mask, results in real-world chaos, war, and political disturbances. In Kali's case, the result was the rapid colonization of India by Britain in the 18th century.
The spirits can influence to whom and how they are passed on to reach a certain place or person, though they will often leave an easily identifiable trail of destruction, making their influence visible to anyone looking for it. Older Masks will be more cautious and are less likely to destroy their hosts. It is even suggested that, through mutual agreement, the host and the Mask can separate amicably with no ill effects. Given the addictive nature of power, however, most Magi presume this is a lie (albeit an unsuccessful one) to dissuade Magi from killing Mask owners. For Magi, possession of a Mask is punishable by death, usually by beheading to prevent post-mortem possession.
That being said, a successful possession or bonding between host and Mask can result in a significant boost of power for the host, and even additional abilities unique to the spirit contained in the Mask. For instance, while a practitioner with a focus on water magic could see an increase in her control and strength, hosting a locus spirit could give her a measure of control over the area the spirit is from if nearby. Host-entity communication does not require physical contact with the Mask, but access to significant power boosts does. The final stages of possession result in the Mask morphing with the host along with other disfiguring changes such as the sprouting of wings or additional limbs, scales, or a myriad of other significant abnormalities. Once that stage is reached, only the death of the host will release the Mask and re-entrap the spirit and is highly advised even by non-Magi, as it risks exposure to the mundane populace.