Dragon Soul Seal

Dragon Soul Magic (竜魂の魔法, Ryūkon no Mahō) is a Lost Magic and one of many, many, many, many, many, many, many, many supplementary magics that involve and Dragon Slayers. It allows the parent dragon to enter their protégé's soul, serving as a defense of sorts until their departure.

Description
Dragon Soul Magic, unlike many other forms of support for the supposed dragon killers, is a purely supplementary magic. It is proof of a parent's love for their child- that undying devotion taking a supernatural form. Dragon Soul Magic works by the dragon abandoning their physical body, compressing all of their physical and magical power into their soul, turning their cells into raw magical particles and compressing it into their soul. The soul is compatible with those who have learnt from the dragon- only a magician whose physiology has been altered by the Factor of the Dragon (竜の因子, Ryū no Inshi), which is simply the magician's body taking on draconic qualities, can accept the soul into their very being. In any case, when the protégé of the dragon accepts their parent's soul, they undergo a unique process which merges their existences.

This merging results in a slew of benefits for the Dragon Slayer. An example of this is by hosting an actual dragon within their existence, the dragon's soul creates anti-bodies against the Dragonborn status for their child, allowing them to use their ever-so-rare Dragon Slayer Magic as if it were simply a normal Elemental Magic with numerous perks instead of the dangerous existence that it actually is. On the dragon's end, it seems that as long as the dragon is inside their child's soul, they can live as long as their child does; though time-related magics and rules such as Jutsu Shiki seem to recognize the Dragon Slayer's age as being a combination of both the parent and child. In addition, Dragon Soul Magic allows the Dragon Slayer to draw upon the power of their parent, giving them sporadic boosts of power. The dragon can also assume control of the human if they are unconcious.

However, Dragon Soul Magic has one great weakness- that of its length. If the child is in danger, the dragon can exit their "child" in a physical spirit form, taking upon a semi-corporal form. Once this is done, the dragon is locked out of using this magic once more, and their time spent as "alive" is extremely limited, with an hour at most allowed; once the time is spent, the dragon will just fade away, turning into raw eternano and dying for good. However, in their last moments, a dragon is capable of giving their child one final power-boost by utilizing the residual, inert Magic Power of their dragon that was once a part of them, a draconian tattoo appears on their arm that serves as the physical manifestation of the aforementioned Magic Power; granting them a drastic increase in power all across the board, if only for an extremely finite duration.

Trivia

 * As some might be wondering, the new revision of the Dragon Slayers introduced in the latest chapter is entirely optional- there would be too much work behind pulling a site-wide revision and several users might not be too happy, so it's not worth the time.
 * Dokkan dokkan paradiiiise