Fusion

Fusion, or Union, is the system in which two or more demons are combined together to create a new demon. A demon created via this system will inherit certain traits and abilities from the "ingredient" demons that were used to create it. The exact nature of this inheritance varies by the types of demons used in the fusion, by the type of demon being created by the fusion, by the phase of the moon (or its equivalent), and by other factors that vary by game.

The general rule for the most basic two-demon fusion is that outcomes are primarily determined by the two races of the ingredient demons, which in turn decide the resultant demon's race. For specific demons of that race, the average sum of the first two demons' base levels are accounted for. For example: a demon of race A fuses with a demon of race B, resulting in a demon of race C at level ((A's base level + B's base level) / 2). But there are many exceptions to this rule.

Some demons cannot be recruited or captured normally, and must instead be created via special fusion. Some of these demons can only be fused once certain prerequisites have been met; for example, some bosses can only be fused once they have been defeated.

In several games, demons can also be fused into weapons (usually swords), giving that gear new abilities based on the abilities of the demon used in the fusion.

Clarification
In Japanese titles, literal Fusion when used, does not refer to traditional Devil Union. The word "Union" is more often used by Atlus Co., Ltd. when unambiguously rendering this concept in English.

Megami Tensei series
In Megami Tensei games, gattai, called fusion here, is possible at a Jakyou no Yakata.

Shin Megami Tensei series
In the main Shin Megami Tensei series, fusions are made at the Cathedral of Shadows. The first three installments of this series include three-demon fusion which gives access to create more powerful demons than in two-demon fusion but with more complicated rules.

Shin Megami Tensei III: Nocturne further changes three-demon fusion to the unique sacrificial fusion mechanic, which is only carried out in a Full Kagutsuchi phase. Nocturne also features a unique mechanic where the fusion chart changes if the protagonist is inflicted with the Cursed ailment from a Magatama; the Cursed fusion chart exclusively results in Haunt, Foul, Wilder, Vile, and Tyrant demons, all of whom are much harder to obtain through recruiting compared to other demons. Lastly Cursed fusion inverts the positive and negative effects of Elemental fusions.

Shin Megami Tensei: Strange Journey onwards adds special fusions using three or more demons. Shin Megami Tensei IV and onward allow full control of what the resultant demon inherits (originally introduced in Devil Survivor). This also includes being able to completely overwrite the demon's starting skills. However, signature skills, like Alice's "Die for Me!" cannot be transferred.

Demons of the same race can be fused to produce Elements, which in turn can be used in fusion to directly produce a child demon of the second parent's race that's either 1 Rank higher or lower (e.g. the neighboring demon in comparison to level.) For example, fusing an Element Aeros with a Raptor Onmoraki (Level 4) will produce a Raptor Zhen that's Level 14 and the next demon in the hierarchy of the Raptor race. On the other hand, fusing Aeros with a Brute Azumi (Level 11) will produce a Brute Daemon (Level 7), which is a weaker demon in the race. Additionally, fusing the strongest demons to produce a stronger demon (and vice versa) allows to cycle between both the strongest and weakest demons of a respective race (such as Femme Kali and Mermaid.)

Shin Megami Tensei: Strange Journey
Fusion in this game is handled through the specialized Demon Fusion Program, which alongside the Demon Summoning Program is sent to the Schwarzwelt crew through a hacked AI, Verne, the brother AI to the crew's Arthur, from an unknown source by The Three Wise Men.

Jack's Squad also attempted to perfect Demon Fusion protocols without the intervention of the Three Wise Men, but their techniques were terribly flawed at best; this, coupled with their lack of the Demon Summoning Program and Demon Fusion Program meant they were forced to enslave demons rather than convince them to join them by negotiation, and that most of their higher level demons were mostly abominations such as Slimes, Pellaidhs, Pisacas, Ghouls or Legions. This would have severe implications for the story as their attempts to fuse demons and humans and abusing demon prisoners came into the fore.

Aside from that, the game also boasts a special fusion technique, which allows for the fusion of more than two demons into a single, more powerful one. However, the ingredients for these fusions are fixed and cannot be exchanged by similar substitutes.

Demons in Strange Journey have a fixed moveset of up to three skills, and don't learn any more moves through leveling up. However, when fusing a demon, it can learn additional skills from its parents based on its inheritance preference. A demon can only learn enough skills to fill five of its six slots.

While fusing, one can opt to use a Demon Source, obtained from fully analyzing demons in their party or stock. Demon Source skills allow the player to impart skills without paying heed to the demon's inheritance preference.

Last Bible series
In the English-language localization of Last Bible (localized under the title Revelations: The Demon Slayer), fusion is referred to as Combination.

Rather than being performed at the Cathedral of Shadows, fusion in Last Bible is performed anyone on the world map using Kishe's Combine spell, and at shrines in Last Bible II.

Majin Tensei II: Spiral Nemesis
Known as REMIX, it can be done at REMIX Stations found at certain spots on the map, which typically requires doing a number of chapters before being able to reach it. The program was developed by Dr. Yagami who hands over the file to Naoki known as FRI-DAY (Factor Recombinant Interpretation Data AnalYzer) after clearing Shinjuku in Tokyo, 1996 AD. It typically appears as a blue human head on the map. The person running the station is, an artificial intelligence in the form of a head. Two to Three demons can be remixed, and multiples of the same demon can be used in a fusion if the player has one on each File. The more powerful races like Deity, Tyrant, Amatsu, etc. require being made through 3-Remix.

Two demons can also be fused on a mission by placing one of them on top of the other. However, it is advised that all equipment be removed from the demons. The equipment is lost after the fusion.

Devil Summoner series
In the Devil Summoner series, demons are fused by Victor at the Gouma-Den. In the Raidou Kuzunoha sub-series, demons can only be used in normal (non-sacrifice) fusion if they have maximum loyalty.

Devil Summoner: Raidou Kuzunoha vs. The Soulless Army
Binary fusion, the fusion of two demons together, is the primary means of fusion at the Gouma-Den. The principles of the fusion will change based on the Orders of the demons used. These will generally have four different outcomes, normal fusion, identical fusion, unstable fusion and inverted fusion. In normal fusion, the Orders of the demons involved will affect the outcome, fusing two demons from different Orders will result in a demon from another Order. In identical fusion, fusing two demons from the same Order will result in a demon from the Skill or Pagan Orders. In unstable fusion, using a Skill Order demon may result in a demon from a random Order. In inverted fusion, using a Pagan Order demon will result in a demon that inherits random abilities, which will override any skills that the demon normally learns, whereas typical normal fusion will not usually allow a demon's natural abilities to be overridden.

Attempting normal binary fusion during a New Moon will not transfer any skills to the resulting demon. Attempting normal binary fusion during a Full Moon will potentially allow a demon's natural abilities to be overridden by the parent demons' skills. In addition to the transfer of skills, fusing demons will result in a demon with slightly superior stats compared to one obtained through Confinement. Each of the two parent demons will increase the resulting stats of the demon based on the yellow pip mark in their stat bars by 1. As an example, fusing Ukobach (1 yellow tally in Luck) with Azumi (1 yellow tally in Strength) will result in a Poltergeist with a bonus to Strength and Luck, but fusing Jubokko with Susano-o, which both have a yellow mark in Vitality will result in an Oumitsunu with only a one point boost to its Vitality.

As demons level up or receive sacrifices, the stat boosts will also be displayed as yellow pips in their stat bar. Fusing these demons will result in potential boosts across all four primary stats, but only by 1 point. The same goes for demons that have received stat boosts through being the result of fusions.

While using one Skill Order demon will result in an unstable fusion, using two Skill Order demons will be overridden by the identical fusion rules. Unstable fusion will result in a random demon with random skills from the parent demons. Using unstable fusion during a Full Moon is an effective way to set up a fusion accident.

Inverted fusion using a Pagan Order demon will result in random skills from the parents demons being passed on to the resulting demon, and in addition, the resulting demon can receive large stat boosts from the parent demons instead of being limited to a one point boost in each stat.

Soul Hackers 2
In Soul Hackers 2, fusion outcomes can be filtered out by specific criteria, such as the resultant race, the increased stats, etc. On the main menu of the facility, several automatically-generated recommendations for fusion are listed.

Persona series
In the Persona series, Personas can be fused in Igor's Velvet Room with the method of fusion varying among the games. Similarly to the rest of the series, fusion outcomes are determined by the Personas' Arcana.

In Megami Ibunroku Persona, fusion requires cards given by demons through negotiation with a maximum of 12 being held at any time. When doing Manual Fusion, White Fusion is ordinary fusion, Orange Fusion indicates a higher chance for fusion accidents to occur, and Blue Fusion indicates the resulting Persona will receive +5 to its highest stat and +20 to Magic Attack. Arrows indicate that a skill will be inherited: a horizontal arrow indicates the resulting Persona will inherit a skill from the first demon used in the fusion, while an angled arrow indicates the resulting persona will inherit a skill from the second demon, with an upward arrow granting +1 to all stats and a beneficial skill and a downward arrow granting -1 to all stats and an unfavorable skill. Fusion can also use items and equipment to gain skills or change stats; using gems will change the result into a Persona of the same Arcana and Totems will give a specific Persona with certain fusions.

In the Persona 2 duology (Innocent Sin & Eternal Punishment), fusion is replaced by summoning which requires numbers of Tarot Cards instead of cards representing their respective demon. To summon a Persona, Igor must be given Tarot Cards matching the Arcana of the selected Persona. Tarot Cards are gained through negotiations with demons in pacts with the protagonists, and Arcana of the cards is the same as the demon contacted. Additionally, Personas can mutate to gain skills or evolve into other Personas.

In Persona 4 Golden, Persona Q, and Persona 5, the protagonists now have control over which skills the fusion result inherits from its components. However, certain skills unique to the Persona (for instance, Alice's Die for Me!) can never be passed on through fusion, and the resultant Personas themselves may be unable to inherit certain types of skills based on their inheritance type. For instance, a Decarabia, with the Fire inheritance type, will never be able to attain any Ice skills from fusion.

In Persona 5 Royal, traits can also be inherited through fusion.

In Persona 5 Strikers, Personas being used as material for a Fusion recipe must meet a minimum level requirement to be able to be used as material. The level requirement varies based on the Fusion recipe. In many cases this requirement will be the Persona's base level, but for particularly strong Fusions the player may be required to level up the Persona, either through EXP or Persona Points. In addition, investing Persona Points into a Persona to increase its stats then using the Persona as a Fusion material will produce "accumulated power", where a portion of the stats invested will be passed down through Fusion.

Persona 3
Listed below is Igor's explanation regarding Fusion in Persona 3 FES.

Fusion Basics

Fusion & S. Links

Types of Fusion

Skill Inheritance

Inheritance Rules

Skill Cards (Portable only)

Incense Card

Fusion Accidents

Item Bearing

DemiKids
All demons fused with the respective partner demon act as Mitamas. Rank UP fusion allows one to fuse two of the same demon together to increase it's HP and MP as well as awaken its dormant skills. Fusing Shiny demons this way with the same demon it's a shiny version of will also result in the Shiny demon increasing two stats, while Boss Type demons can increase their stats from anywhere between 1 to 4. Certain demons who are fused this way a number of times will Rank Up and become a new demon. Such as fusing multiple Cupids together will evolve them into Nikare.

Relic Fusion is a special fusion where you take fossils that can be bought in the game and fuse three of them together. This can result in either a zombie version of certain demons or a regular demon being born. No matter the order the demon will be the same. Resurrecting Zombie fusion will result in a zombie demon being brought back to life after its soul is fused with it.

Devil Survivor series
In all games of the series, demon fusion takes place using the Cathedral of Shadows Program (often termed cath.exe) that is introduced with the Demon Summoning Program. In all games to date, only two demons can be used for fusion. Unlike previous games in the entire Shin Megami Tensei series, the player has control over what skills the resultant demon inherits from its fusion components, and can even search what demons are required to produce the desired result. If the fusion components have increased stats (either from leveling up or prior fusions), the resultant demon can also inherit a portion of those stats (albeit distributed somewhat randomly).