The Human Fusion Spell, also known as the Fusion Spell, is a magical spell that merges two souls into one. The one the spell is being used on causes their body to turn into powder and their soul begins to merge with the spellcaster's soul.
The only conjurors who are sucessfully skilled at practicing this spell are the two Merlineans, Balthazar Blake and Veronica Gorloisen. This spell is used by Veronica to save Balthazar from the evil Morgana le Fay, who, centuries later, was destroyed by the Prime Merlinean and her plan of unleashing the spirits of many dead evil spellcasters via the sinister ceremony known as The Rising was foiled.
Process and UsageTo use the spell, a sorcerer starts by concentrating their magic while pointing their hands toward their target, and then pointing at themselves as seen when both Balthazar and Veronica uses this spell. After this, the target will have their soul extracted from their body and enters the caster's. At the same time, the target's body is also absorbed as well, though the process would reduce it to electron dust, as was when Morgana exits Balthazar's body, she ended up in a ghostly form of her former self.
It seemed that judging from how Morgana's soul possessed Veronica and then Balthazar for a brief amount of time after using the Fusion Spell, shows that the extracted soul was not merged into the caster's like the name of the spell implied, but rather possibly sealed within their body and soul like a containment vessel. This was supported by the fact that Balthazar was able to extract Morgana's soul without extracting Veronica's, which also shows that when using the spell, the caster instead points both his/her spirit and their intended target's soul. Not only that, the extracted spirit will threaten to possess the caster once they inside their body.
If one's soul whom extracted by the Fusion Spell breaks free from the caster's body, as seen when Morgana came out from Balthazar's, they inevitably became a ghostly form of their former self prior to affected by the Fusion Spell, consisted as mass of electron dust that forms what has left of their physical form, includes any clothing, catalyst/s, and any accessory they wore when attacked with fusion spell. In this form, they presumably invulnerable with physical attack, only weak against magical ones. However, they can avoid any magical attack by simply dissolve themselves. Since their non-corporeal body consist of electron dust, victim of Human Fusion Spell can only rendered solid with torrent of electrical blasts, which resulting them more vulnerable that should be barraged with powerful spells, he/she will dissolve into nothing and dies, as their non-corporeal body extinguished by the attack. Plasma Bolts seemed to be the best spell to finish the vulnerable non-physical form of Fusion Spell victims.
As Morgana is the only known magic caster that killed in this method, it's unknown whether alternate methods exist.