A Guide to Possession

Possession is an iconic ability of demons and devils in religion, mythology and fiction. Even Lovecraft's starspawn could get up to it every now and then. DnD 5e is sorely lacking in this respect, favoring demons that are rather more fleshy. This is a guide to a ruleset for possession that's fun, satisfying, and entertaining for Dungeons and Dragons 5e.

A creature with the ability to possess vacates its corporeal form to enter a victim, if it has a corporeal form to begin with. For those with corporeal bodies, this may mean phasing into an incorporeal state entirely, or it may involve actually abandoning its flesh as if dead. Fiends and aberrations have a different relationship to their flesh than those beings of the Prime Material, and new flesh is almost always available for a clever monster.

The entity gains the ability of Possession, which raises its CR by 1 and potentially adds a whole new creature to a challenge - the host.

Entities with the ability of Possession have a vice and abhor at least one virtue, which are important to the ability. The Possession progresses when the host is coerced of their own free will to indulge the entity's vice, and the Possession is fought by confronting the entity with the virtue that it abhors.

For the purposes of this ability, there are four virtues and four vices. The vices are: Inaction, Selfishness, Greed, and Spite. The four virtues are Vigor, Selflessness, Composure, and Serenity. They will be explained later. Additionally, if the entity stands opposed to a particular deity, philosophy, organization, etc. then striking a blow against their enemies can act alongside their vice, though they are particularly vulnerable to exorcism when one of their chosen enemies is involved.

There are four stages of Possession: Infestation, Oppression, Obsession, and Possession. All begin with Infestation and the final goal of all entities is Possession. Each stage comes with a dice pool and a set of abilities with which to effect or torment their host. Possession progresses from one stage to the next when the host indulges in the entity's vice of their own free will. Progress is halted temporarily by engaging in the virtue that they abhor. 

The Four Stages of Possession

There are four stages of Possession. Each stage has a die pool that can be used to effect the host and the environment around them in the hopes of securing a tighter grip on their souls. Every day at the darkest point of the night, the entity rolls all of their remaining die pool, and their die are replenished as indicated below. If the result of the roll exceeds the host's Wisdom score, the Possession advances to the next stage. To use the die pool, either choose from the associated table or roll the die. Any damage done uses one die from the pool and adds the entity's Intelligence modifier, and any save DC is 10+the entity's Charisma modifier.

Infestation - Die Pool 1d4

In an infestation, the incorporeal entity has not penetrated much deeper than the skin of their host. Their power is limited in this state, and they must be careful not to be caught out as they are vulnerable to even mundane spiritual intervention. A successful Wisdom (Religion) check against the entity's DC by even a layman could exorcise the entity, ending the possession. Canny entities that survive such an attempt may attempt to hide, leaving the host to their own devices for a time before resurfacing with a vengeance after the host has a particularly Vice-ridden day.

Oppression - Die Pool 2d6

An oppression is when the host can no longer deny that something is wrong. They may not have realized that they are possessed, but they know that it is no mere illness. At this stage, only a paragon of faith (as in a Cleric) can attempt the exorcism. Doing so requires a Wisdom (Religion) check against the entity's DC. Each successful attempt removes one die from the entity's pool. A failed attempt deals 1d6 Psychic damage to the exorcist or to the host, at the entity's choice. The entity attempts to grind down the host's will at this stage, reinforcing the hopelessness of the situation.

Obsession - Die Pool 3d8

An obsession occurs and the host now knows for certain that they are in deep supernatural trouble. They likely know the nature of the entity possessing them, which sort of Hell, Abyss, or Far Realm it was spawned in, and what it wants to do to their body and soul. They spend ten to fifteen minutes at a time several times a day staring into space, lost in thoughts of their own damnation and the bitterest sins of their companions. The entity will spend this time 'closing the deal,' snuffing out the last pieces of their resistance. It will seek to fully isolate the host, directing violence towards their friends and family until there's nothing left for them to turn to. Only a Cleric capable of casting 3rd level spells can perform the exorcism at this stage. Doing so requires a Wisdom (Religion) check against the entity's DC, and the check is performed at Disadvantage. Clerics typically have an assistant or younger cleric aid them in the check to avoid the Disadvantage. Each attempt deals 1d8 Psychic damage to the Cleric, an assistant, or the host, or double this on a failed attempt.

Possession - Die Pool 4d10

At the fourth stage of possession, the entity is in full control of the host for hours on end, sometimes entire days. They turn their attention to other plans and schemes, use pieces of their host's soul to summon more entities, and hunt down representatives of divine powers opposed to them. They may start cults, go on murder sprees, or even simply contribute to organized crime and amass large stores of material wealth to satisfy their greed. Only a Cleric capable of casting 5th level spells can perform the exorcism at this stage. Doing so requires a Wisdom (Religion) check against the entity's DC, and the check is performed at Disadvantage. Clerics typically have an assistant or younger cleric aid them in the check to avoid the Disadvantage. Each attempt deals 1d10 Psychic damage to the Cleric, an assistant, or the host, or double this on a failed attempt. At this point, the entity will use every means at their disposal to prevent being dislodged, including using their host to physically attack the exorcist. Additionally, the entity may act out of spite and attempt to kill their host to avoid returning to their home plane empty handed. A host that dies under full possession is bound to the entity, and any attempt to resurrect them has a 50% chance of resurrecting their body with the entity in full control while the host's soul remains in the afterlife.

Virtues or Vices

Every entity has one Vice they align with and one Virtue that they abhor. If the host indulges in the entity's Vice, the entity gains one die for their die pool. If the host successfully engages in the Virtue that the entity abhors, the entity loses a die from their pool.

Vices

Indolence - The vice of Indolence has been indulged when the host could do good for themselves or others and decides to simply not do that. This can be as simple as forgoing to cook a meal and instead eating it raw to the detriment of their health to passing by a wounded merchant on the road to refusing to accept one's own flaws even when they harm another. Any time a failure to act, mentally or physically, has resulted in harm, Indolence has been committed. Entities of Indolence are sloppy, messy types that don't like to put too much effort into their torments. They favor straightforward goading, punishing their hosts for taking actions and simply leaving them alone when they sloth about. Indolence encompasses Sloth and Pride.

Indulgence - The vice of Indulgence has been indulged when the host prioritizes their own needs and desires to the detriment of others. They fulfill their needs and leave others wanting, hurt or ashamed in the process. Eating a full meal when supplies are low when the rest of the party is on half rations, picking a pocket, pursuing a romance when people are counting on the host's assistance and sneaking out the window in the morning after are all examples of Indulgence. Entities of Indulgence are tempters, normalizing the most lurid behavior and reminding their host about how much fun they could be having. Indulgence encompasses Lust and Gluttony.

Covetousness - The vice of Covetousness is fairly straightforward. When something has been taken that is better used by someone else, Covetousness has been indulged. This could be the host taking more than their fair share of party loot, haggling down the price from the town's merchant or haggling up the price of the junk they found in the nearby dungeon. Entities of Covetousness prefer to tempt their host with zero sum mind games, pointing out how much GOOD they could do with the ill-gotten gains in front of them. Covetousness covers Greed and Envy.

Truculence - The vice of Truculence is the vice of revenge, destruction, wrath and ruin. Examples of Truculence could be revenge against grievances real or imagined, vandalism, social attacks, and of course murder. Entities of Truculence express outrage at the actions of others, try to trick their hosts into attacking allies or innocents, threaten and coerce their hosts with painful scratches or promises of extra-horrific and creative damnations.

Despair - The vice of Despair is a negative trait. It's an inability to connect with virtue, be it through a fundamental disconnect with goodness and positivity or through, as the name suggests, a soul-eating despair that causes one to give up on their own virtues. Acts of Despair include a refusal to exercise ones' virtue, preventing another from acting on their own virtues, and attacking an enemy where their virtues may make them vulnerable. Entities of Despair spread across the souls of their hosts like a mold or a tumor, slowly rotting away anything good about them. They drink the joy out of their host's life, diminishing rewards for virtue.

Virtues

Vigor - The virtue of Vigor is mental and physical doggedness. When an innocent needs a protector or when work must be done then the Vigorous step in. Entities that abhor Vigor are usually craven, prone to shrinking back when confronted directly and plotting to undermine their host's best instincts to prevent them from decisiveness. Vigor encompasses Charity and Diligence.

Kindness- The virtue of Kindness is self-explanatory. The Kind are good citizens, treating their fellow man like family. They know when to act fiercely to protect their society, family, country, faith, etc. and they know when to apply the soft touch of Mercy. Entities that abhor Kindness are misanthropic isolationists and push those views onto the host. Kindness encompasses the complementary virtues of Justice and Mercy.

Composure - The virtue of Composure is stoicism, calculated action, focus on priorities and principles. Acts of Composure include solving puzzles, resisting temptations, ignoring insults, shrugging off influence, and remaining calm in a crisis. Entities that abhor Composure are often brash and seek to imbalance their host. They often find interrupting sleep to be helpful to their aims. Composure encompasses Patience and Chastity.

Serenity - The virtue of Serenity is one of contentment, joy, and optimism. Acts of Serentiy include forgiveness and mercy on one's enemies, kindness towards others, and exercising the wisdom to know when a situation cannot be fought against. Entities that abhor Serenity are often excitable, jabbering things that enjoy nothing more than to get a rise out of their hosts, frustrating them as often as possible. Serenity encompasses Humility and Patience.

Courage - The virtue of Courage is a negative virtue. It's the power to combat the evils and weakness in ones' own soul. It's the strength to prioritize good and rightness over fear, doubt, and wrong. Acts of Courage include denying ones' vices, assisting another in overcoming theirs, and seeking to overcome ones' enemies without the use of trickery or preying on their weaknesses. Entities that abhor Courage are fearsome things that belittle and sew doubt. They use the host's fears and attack the things and people that they love.

Other Notes

Exorcism

Exorcisms take one hour per attempt and degrade the entity's die pool. The entity is free to act out with any and all powers during an exorcism, but doing so may cause its die pool to degrade faster. When the die pool reaches zero, the possession is downgraded by one level. If the level is at an Infestation during a successful exorcism, the entity is driven back to their home plane.

Multiple Entities

Entities can pile in on the same host. If the entities are aligned, they may cooperate or they may fight for dominance. Either way, each possessing entity maintains their own die pool and follows their own agenda. A Cleric capable of casting 5th level spells can exorcise them all at once, but otherwise they must be removed in turn. Hosts rarely survive being used as a hive for extraplanar entities. If one entity is dominant, it can control the die pools and actions of its subordinates.

Good-Aligned Possession

Possession by good-aligned beings is possible, though rare. In this case, reverse the effects of their Vices and Virtues, as they will be empowered by a specific Virtue and abhor a specific Vice above the others.

Own Free Will

There are abilities the entity gains in the later stages to take full control in various ways. They cannot puppeteer their host into engaging in their vice in this way and derive the benefits. That does not mean the host cannot be tricked. If an Entity of Truculence were to, for instance, use their Minor Illusion ability to cause their host to hallucinate that the local merchant is their worst enemy, then the assault and murder that might follow on from that trick is still indulging the sin. The important thing is that the host chose the action, not that the host knew the context within that action. That isn't to say that the Entity couldn't take full control and murder the merchant, leaving the host to wake up in the midst of the carnage left behind to deal with the mess and perhaps fight their way out.

Chosen Foe

Some entities (Demons and Devils in particular) stand in opposition or rebellion to gods, angelic choirs, religious sects, or even some Prime Material organizations (Such as a Demon in service to Lolth opposing an elven kingdom). When they strike a blow against such an organization, they gain a die for their pool and all damage they deal with their abilities is doubled for the rest of the week. During an exorcism, if one of the participants is a member of that chosen foe, they remove two dice instead of one.