Celestial Cross Fuse

Celestial Cross Fuse (星歌・一心同体 (セレスチアル・クロス・ヒューズ), Seresutiaru Kurosu Hyūzu lit. Celestial Elegy: Two Hearts Beating As One): A unique spell; not a magic proper, that makes full use of a Celestial Spirit Mage's link with their spirits.

Description
Due to the user holding a contract with them, the sprit has a special "magic link" with the user, essentially a contract of sorts that they can annul at any time. By synchronizing their magical power frequency with that of the spirit's when the two are in close proximity, with the spirit's approval, the user can perform this spell; which merges the mage and the spirit, integrating the spirit within their body. This allows the mage to replicate the powers of the spirit; filtering out magic (known as drawing) from the spirit 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 spirit's magic. When the mage and their spirit are merged, two kinds of beings are brought together, and since the user possesses a comparable consciousness, the spirit 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.

The merger grants the user an extreme boost in power; giving them access to new abilities, including executing powered-up spells; and when entering fusion mode, a pair of white feathered wings will emerge from the user's back. 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. 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. The mage can always initiate telepathic contact with the spirit. However, in the case that that the user loses consciousness or the spirit for any other reason has to assume full control over their shared body the spirit creates a so-called "Internal Space" (内部空間, Naibu Kūkan, pocket dimension), where the user can communicate with the spirit in safety and even recuperate.

Spells

 * Apostle Cooldown (使徒・冷え込み, Shito Hiekomi): In a partial use, Celestial Cross Fuse completely drains the user's full magical power to recover from a foe's attack and slow down the opponent for six seconds, or until a spell is landed. The duration of this period can be adjusted by the user's magical power levels. During this time the user's attack cooldown is reduced to nothing, allowing the user to link together several powerful spells one after another; even spamming what would be considered spells with a normally long casting time. Also, if the opponent is evading, the user can still continue their assault against them, as the foe doesn't receive temporarily invulnerability during this method.

Trivia

 * Celestial Cross Fuse was created before Lucy gained Aquarius' Water Magic, so that is why the concepts are similar.