There are several instances of Possession being cited in Anima (most notably the possession of Gunnar Skulinbert by Bloodbane, but there are other cases through Gaia). I decided to write down rules to allow characters to be possessed (or creatures to possess characters...it's always a matter of points of view!).
Possession is a process known through Gaia, consisting in a being betweeen worlds to enter the body of a mortal, either mixing the consciousness, physical characteristics, and power or completely overthrowing the host, turning it into a mere vessel. Although at times powerful entities use possession of their own will to act into the world in a disguised way, there is also the possibility for summoners of exploiting the “Possession” process to turn themselves into bound creatures or to bind godly powers into mortal beings. Of course such processes, especially the second one are extremely dangerous as a miscalculation can have the soul of the host destroyed by the “pressure” of the possessing entity.
Normally, only Spiritual Beings can enter physical hosts and possess them. Nevertheless, beings with high Gnosis 35 or more can also try to overtake another body, regardless of their nature. In order to do so, they must enter the physical body of the host, which must take a PsR check against a difficulty equal to the entity Presence plus its Willpower bonus, in order to resist it. If the entity’s Gnosis is higher than the host’s by 20 points, double its Presence, if its higher by 40 points or more, triple it. If the possession is successful, unless the creature and host had a previous accord of some kind, the two minds will struggle for control over the body, hence they make an opposed Willpower check. The host will suffer a penalty to the Willpower check of -1 for each 10 points of Failure Level in the previous check, rounded up. The possessing entity will gain +1 to his Willpower check for every 5 points of difference in Gnosis between itself and the host up to 15Gnosis points of Difference, than an additional +2 to the Willpower check for each additional 5 points of difference in Gnosis. If the difference between the two Willpower checks is between 0 and 3, the two consciousness will both be present and entwined. Unless the two reach an understanding of sort, the character will suffer from an All Action Penalty of -150 and will have to repeat the check the following minute. If the host wins the Willpower check by a difference of 4 to 10, the possessing consciousness will be put to sleep for a number of hours equal to the difference. If the host wins the Willpower check by a difference higher than 10, he/she manages to expel the spirit. If the possessing entity wins the Willpower check by a difference of 4 to 10, the host’s consciousness will be put to sleep for a number of days equal to the difference. If the possessing entity wins the Willpower check by a difference higher than 10, the host’s consciousness will be put to sleep for as long as the entity wishes.
BOUND CREATURES POSSESSION:
Summoners can choose to become possessed under controlled conditions by having previously bound and controlled creatures enter their body. Only creatures which were successfully controlled and bound somewhere can possess the Summoner following these rules. Familiars and Sheeles cannot possess their master due to some inexplicable paradox (each time such feat has been attempted in history the results were extraordinary variable, usually resulting on either the breakage of the familiar bond and/or the “reabsorbing” of the Shelee within the Summoner). In order to become possessed by a controlled bound creature, the Summoner must take it out of its container and transfer it automatically to his/her body. Unless the Summoner and the creature are willingly cooperating, the creature has a chance to overtake the Summoner and/or break free from all summoning bonds. The two take an opposed Willpower check as described in previous chapter, but the summoner can apply a bonus of +1Willpower for each 10 points by which he/she passed the last Control check made on the creature. If the summoner is successful (winning the check by 4 or more points), the creature remains controlled and bound for as long as the Possession lasts. This kind of forced Possession can only last for a number of minutes equal to the sum of the Summoner’s Power and Willpower values, after which the Possession cannot be used again for 24 hours. If the creature wins the check by 4 or more points it is immediately freed from any Control and/or Bind ability the summoner might have previously used on it and can choose to either control the possessed summoner or immediately separate from him/her.
Summoned Possession happens when a Summoner obtains a stable possession of an individual (including himself) from another powerful entity. In order to do so, the Summoner must pass a Summon and Bind check on the intended entity and spend Zeon for both rituals. A creature with Gnosis higher than 45 can choose to refuse being summoned as normal, but the process ignores Gnosis limitations to Binding. When the creature is summoned for a Possession, the opposed Willpower check for Possession will occur as normal, but the Summoner can choose to grant a +1Willpower to any of the two parties involved in the Possession for each 10 points by which he/she passed the Summon check.
Possession grants extreme power to both the possessed and the possessing entity, but is highly risky for the host, as containing more than one essence in one’s own body and soul can easily tear them apart. The Possessed being can choose the best Physical Characteristics out of those of the two beings and can modify its size in accordance to it, and uses the Spiritual Characteristics of the one in control of the body. The Possessed can only use Attack, Defense, Projection, Knowledge and Elan based Abilities of the mind controlling it, as modified by any improved characteristics, while the base of all other Abilities is that of the host. The Possessed adds Ki, Ki Accumulation, Zeon, MA, Free PP and Psychic Potential of the possessing entity to his/her own, and benefits from any Essential Ability and/or Creature Power owned by the entity, as well as benefitting from the best Gnosis out of the two. The appearance of a Possessed being can be extremely variable at Master’s discretion with the possessing entity being completely hidden or manifest or only partially altering the appearance of the host.
As for side effects, each time a possession is interrupted the host must pass a PsR check against a difficulty equal to the possessing entity’s Presence, which is doubled if the entity’s Gnosis was more 20 points higher than that of the host, and tripled if the entity’s Gnosis was more than 40 points higher than that of the host. If case of failure, the host faints to unconsciousness for a number of hours equal to the failure level. If the Failure Level is higher than 40 the character additionally suffers from a permanent loss of 1 point from anyone random Spiritual Characteristic for every 10 points of Failure Level beyond 40. If the possessing entity’s Gnosis is higher than 30, the same check must be taken once per hour of Possession and if the entity’s Gnosis is higher than 40 once per minute. A host will not faint during the Possession, but permanent loss of Spiritual Characteristics may happen. Additionally, if the entity that possessed the host wasn’t a Spiritual Being, the host must take a PhR check against the same difficulty each time he/she takes the PsR check. The host suffers Life Points loss equal to Failure Level and on a Failure Level higher than 40, he/she will additionally suffer from the permanent loss of 1 point from anyone random Physical Characteristic for every 10 points of Failure Level beyond 40. Life points lost in this process are recovered at a rate of Life Sacrifice. If anyone characteristic ever reaches 0 due to this process, the host instantly dies, and his/her soul is destroyed.
In certain occasions, a host may be particularly keen to the entity which is possessing him/her (such as someone with Kagutsuchi’s Blood being possessed by a Fire Elemental, or someone with a Beryl’s Avatar’s Blood or high Elan level with a Beryl being possessed by a Light Elemental). In such cases the Game Master can choose to either entitle the host a bonus to PsR and PhR checks for enduring/interrupting a Possession, treat the host as having a higher Gnosis for calculating the PsR and PhR checks difficulties, and/or delay the PsR and PhR checks for hosting entities with higher Gnosis. Once again, Possession is an extremely variable and unpredictable process, and bonuses granted by Possession Affinity can differ greatly depending on host/entity combinations. The following list is only for quick references but should be used at Game Master’s discretion.
Easily Possessed Disadvantage: Grants a bonus of +30 to all Summoning Abilities checks related to Possession, -30PsR to checks related to Possession, +30PhR to checks related to Possession, -3 to Willpower checks related to Possession.
Blood Legacies: Grants a bonus of +10-30 to all Summoning Abilities checks related to Possession, +10-30PsR to checks related to Possession, and a +10-30PhR to checks related to Possession. These bonuses apply only when attempting a Possession from an entity which nature is close to that of the entity that originally sealed the pact with the ancestors of the character and vary depending on how close the Possessing entity is to the one that dealt the pact.
Elan higher than 50: Grants a bonus of +10-30 to all Summoning Abilities checks related to Possession, +10-30PsR to checks related to Possession, and a +10-30PhR to checks related to Possession. These bonuses apply only when attempting a Possession from an entity which nature is close to that of the entity that grants Elan to the character and vary depending on how close the Possessing entity is to the one that dealt the pact.
Elan 100: Grants a bonus of +50 to all Summoning Abilities checks related to Possession, +50PsR to checks related to Possession, and a +50PhR to checks related to Possession. These bonuses apply only when attempting a Possession from the entity which grants Elan to the character.
Racial Adjustments: Grants a bonus of +10 to all Summoning Abilities checks related to Possession, +10PsR to checks related to Possession, and a +10PhR to checks related to Possession. These bonuses apply to members of different Races as well as Nephilims, and only when attempting Possessions from creatures with a specific nature, as indicated below:
Sylvain – Light Elementals.
D’Anjaini – Illusion Elementals.
Ebudan – Air Elementals.
Jayan – Lightning and Earth Elementals.
Daimah – Essence Elementals.
Duk’Zarist – Darkness and Fire Elementals.
Devah – Creation Elementals.
Vetala – Blood Elementals.
Tuan Dalyr – Destruction Elementals and Great Beasts related Creatures.
Dvergr – Dvergr inflict -30 to all Summoning Abilities checks related to Possession, have +30PsR to checks related to Possession, +30PhR to checks related to Possession, +3 to Willpower checks related to Possession, regardless of their Clan.
Aachiince – Insectoid Creatures.
Fafnr – Dragon related Creatures.
Homunculus – Homunculus cannot be possessed.
Okami – Tsukiyomi related Creatures.
Eldeofol – Ice and Void Elementals.
Trold – Water Elementals.
Star Children – None.
Risen – Undead Creatures. Cannot be possessed by other Creatures.
Fae – Magic related Creatures.