Heroic Blessing is a hieromancy spell described in Monster Girl Encyclopedia World Guide III: Sabbath Grimoire.
Guide Book Entry
A hieromancy spell that infuses the target with magnificent divine mana, causing him to awaken to his power as a hero. It's an extremely advanced spell that uses vast mana. Only the gods themselves, the angels who are their messengers, and a handful of saints chosen by the gods can use it. The function of this spell is to “call out to a god, connect the target to a god, and arrange for a smooth transfer of mana.” Even when angels and humans use this spell, it is not the caster who bestows power upon the target, but the god, and ultimately it is only the “god”, the one herself who is actually granting the power, that chooses who is to become a hero. Hence, casters are unable to change targets into heroes just by using this spell, as they cannot become heroes without the approval of a god.
Those allotted a portion of the tremendously vast mana possessed by a god via this “heroic blessing” become heroes, which results in their abilities increasing by leaps and bounds. They are endowed with such high physical abilities that it would normally be impossible with only a mere human body. Due to the abundance of mana, they wield advanced spells and exhibit the strength of one thousand warriors, becoming exactly like legendary heroes. There are a number of enhancement spells that can boost magic power and physical abilities such as “miracle of power”, but the power that can be gained by becoming a hero is so tremendous that those pale in comparison, and the mechanism for how that power is acquired is also a fundamentally different thing than enhancement magic.
The essence of this spell is not to “augment or amplify abilities” like ordinary enhancement magic, but instead “to raise the limit of abilities”. By nature, all living things including humans have a limit to how much mana they can hold within their body. Ergo, there is also a limit to how much power a human can possess, but this spell raises that limit for the target by leaps and bounds via divine mana, and it can make him evolve into a superhuman being. On top of that, those who have had their power growth spurred by the divine blessing are the beings called “heroes”. In other words, unlike “enhancement magic”, which can only temporarily augment abilities, the power acquired from becoming a hero is ultimately the result of that person's very abilities themselves which have grown beyond the limit after the fruition of that person's efforts and the blossoming of his talents. Hence, not just anybody can become a hero, and neither is their excellence the result of having become a hero. In fact, only those who possess such extraordinary qualities that they can transcend human limitations and those who have made tireless efforts, continuously increasing their own power, can become heroes. In this way, the power of a hero is an entirely different thing than the temporary power obtained by means of enhancement magic.
Since the evolution from human to hero is a permanent and irreversible thing, it cannot be counteracted by “spell break”, and even if the hero were to lose the god's blessing, the increased power obtained from having become a hero would not be lost; however, if the god's wrath had been incurred, the god's blessing would turn against him, and all of the divine mana remaining in the target's body would horribly debilitate him, so he'd end up in a condition as if he had lost the power of a hero. In that case, it's possible to recover one's power as a hero by somehow expelling the divine mana from inside the body, or by monsterizing or incubizing, eroding the divine mana into mamono mana, and absorbing it completely as one's own power.
Many of the heroes currently in existence are those who were granted blessings by the chief god, but there are also those who became heroes by receiving blessings from other gods such as the goddess of love, “Eros”, or the “fallen god” worshiped by monsters. Gods make their choices of heroes based on an evaluation of the abilities possessed by the target, the target's mentality, qualities, and so forth, and basically those who conform to the will and dogma of the god sharing power are granted power as heroes.
Also, by becoming a hero, the direction in which power growth is spurred is determined to some degree based on the the god's doctrine and the aspect over which she rules. In the case of heroes made by the hand of the chief god, she tends to mainly produce superior warriors and magicians with the objective of fighting monsters, or superior healers to treat the wounded in battle. On the other hand, the power acquired by becoming a hero is not just physical abilities and mana.
By receiving the blessing of the goddess of love, “Eros”, sensitivity and expressiveness are honed, and one can become a songstress who inspires emotions filled with love and hope in people, able to revitalize people's hearts just by singing, and worshipers of “Hephaestus”, the god of smithing, acquire superhuman concentration and surgical precision, becoming craftsmen who create weapons and armor that are even favored by the gods. Such individuals are also “heroes” blessed by the gods. However, in human society, the term “hero” is only used to describe “heroes who have received the blessing of the chief god and fight monsters”. Artists, craftsmen, and so on, who aren't involved in fighting are not treated as heroes regardless of whether they have the power of a hero or not. Also, of course, incubi and monsters who have received the blessings of gods who favor monsters are not treated as heroes either.
Furthermore, in the case of the chief god, the birth of most heroes is also accompanied by an oracle or a hero ordination ceremony performed by the Order, and even when such functions are skipped and the god directly confers power upon heroes, the god's voice instructs the heroes themselves about their mission in most cases, so most of the chief god's heroes are self-aware of the fact that they are heroes.
On the other hand, as for the other gods, if it seems that a human they're interested in “is about to reach the limit of what is humanly possible should the current course of growth continue”, they may sometimes awaken that person's power as a hero without any ceremony or divine revelation, and in such cases, that person will usually not be self-aware of the fact that he had become a hero.
“In the name of god, let us confer heroic power upon thee.
And by god's commandment, I shall constantly be with thee...
Always... No matter what the time...♥”
~a valkyrie who guideth a hero~