Junction

Junction (人魔一体 (ジャンクション), Jankushon lit. Human-Demon Synchronize) is a unique spell which allows a demon and their human host to merge. It can only work if the demon has a special "magic link" with the user, essentially a contract of sorts that the user can annul at any time.

Description
If the user synchronizes their magical power frequency with that of the demon's when the two are in close proximity- or in some cases, when the demon is sealed inside of the host, they can activate it at any time- with the demon's approval, the user can perform a junction; which merges the user and the demon, integrating the demon's powers within their body; though this process can take time even if their bodies have fused (and it sometimes just freezes if the two are resisting assimilation). This allows the host to replicate the powers of the demon; filtering out magic (known as drawing) from the demon and storing it ready to be used by the user. This allows the user to possess near superhuman abilities, such as increased speed and strength, as well as being able to use the demon's magic. The merger grants the user an extreme boost in power; giving them access to new abilities, including executing powered-up spells; and the merger can only be executed when the user gathers enough eternano; however. In order to activate the merger, the spell has an "limit meter" that fills as the user collects eternano; which are glowing orbs of light that appears whenever an attack is landed or an attack is blocked; the user automatically absorbs eternano as they move around the area, but not while attacking or being attacked. When the user can enter the merger, the user begins to glow orange. Once the merger is entered, the user's magical power slowly drains, and the merger ends when it is depleted. The rate at which the power drains can be lowered by using certain accessories and equipment pieces. While in the merger, the character can still collect eternano, refilling their power as it depleted. While in the merger, the user slowly begins regenerating their stamina; while they gain a higher chance of obtaining a critical hit; as well as generic bonuses as special attacks, increased damage, and other enhancements.

Entering the merger blocks any spell the opponent is striking the user with and knocks them back; however, this can be a double-edged sword, as the user's current magic is cancelled out when activating the mode; but despite this, this drawback can be morphed into a blessing, letting attacks be chained together in new ways When demon and host are merged, two kinds of beings are brought together, and since the user possesses a comparable consciousness, the demon will lose their solid form and can only manifest for limited periods of time while the merging is occurring; giving the user superhuman strength and enhancing their body functions. During a successful merge, striking hair or pupil colour changes become apparent, and it will even change the colour of clothing the user is wearing at the time- perhaps at higher levels of power, the user(s) will undergo a drastic physical change. The demon can always initiate telepathic contact with the user. However, in the case that the user loses consciousness or the demon for any other reason has to assume full control over their shared body the demon creates a so-called "Internal Space" (内部空間, Naibu Kūkan, pocket dimension), where the user can communicate with the user in safety and even recuperate. The factor that determines whether or not a demon and their host can Junction is known as the Synchro Rating; a person with a higher rate is more likely able to perform a Junction than one who has a low rate. If a demon and human are not getting along and always arguing, Junction is not possible. The human and demon must be in perfect harmony in order for the transformation to take place.

Trivia

 * Junction is partly based off of Fusion from Dragon Ball Z, but mostly from the Zoid/Organoid fusion from the Zoids franchise.