Despite being heavily steeped in the setting of Archanea and to a lesser extent Jugdral and Tellius, the place of the central antagonist of Fire Emblem Awakening, the Fell Dragon Grima, in the wider setting was unclear. His origins are never explained in the game and the best thing we had for a long time was a passage from the Art of Fire Emblem Awakening:
"According to oral tradition, Grima took the form of a huge dragon who may have been a descendant of a tribe of earth dragons who had a very advanced culture on the continent of Archanea tens of thousands of years ago." -The Art of Fire Emblem Awakening (page 213)
This suggests that Grima may be a type of Shadow Dragon (like Medeus turned into during the War of Liberation and in New Mystery of the Emblem and Loptous was heavily suggested to be as well), given his association with darkness. However, art and databooks such as these are not the best source of information as they are prone to mistakes and inventing information. In the case of this book for example, it is suggested that Tiki named the Fire Emblem of Archanea/Ylisse as such (page 215). This is never mentioned in Awakening and it is also obviously incorrect to anyone who has played any of the Archanea games.
Awakening also implied there was more to Grima than being a Earth or Shadow Dragon, since it included a disturbing detail in his design that is only revealed when he is low on health. Behind a plate of what is presumably exoskeleton lurks a gigantic, disturbing human face:
A mysterious creature with unclear origins and two faces thus. It was not until 5 years later that answers were given about the origins of Grima in Fire Emblem Echoes: Shadows of Valentia. In the post-game, Alm and Celica venture into the depths of the lost city of Thabes (a location that also appeared in Shadow Dragon and New Mystery of the Emblem) and find tablets detailing the story of an alchemist named Forneus. He used his own blood and that of a Divine Dragon to create a perfect being in his workshop, but the being overtook his thoughts and drove him even more mad than he already was. At the bottom of the Thabes Labyrinth, it is revealed that this creature is Grima, though in this game he is only called "The Creation". He is defeated by Alm and Celica's army, but afterwards the following message is displayed:
"The horror has passed. Yet still a foul air lingers, filling this place with a sense of dread..." -message after defeating the Creation in Shadows of Valentia
This of course foreshadows the return of Grima a thousand years later during the Schism (the backstory war of Awakening) and Awakening itself.
This backstory contextualizes Grima a great deal and gives him a proper place in the wider lore of Archanea. It also explains why he has a large human face: he is part human. However, one question does remain: where did the Divine Dragon blood come from?
One detail in Fire Emblem Awakening that stands out is that Naga and Grima share their Awakening ceremony. The Fire Emblem or Binding Shield is needed to revive Grima as much as it is to summon Naga's spirit, even though the object is only ever tied to Naga in the lore. It was made by Naga from one of her fangs to keep the Earth Dragons sealed in the Dragon's Table asleep and the gemstones in it are tied to Naga as well according to Awakening:
"Yes, there are five of themβArgent, Sable, Gules, Azure, and Vert. Each holds a portion of Naga's power." -Tiki in Chapter 16 of Awakening
A connection is suggested between Grima and Naga, but once again Awakening fails to elaborate on this. However, Shadows of Valentia might give the answer. It is notable that Grima was created in Thabes and his creator, Forneus, also lived in Thabes. This city in the middle of the desert is the oldest known settlement in all of Archanea/Ylisse. According to the timeline of Archanea that was first given at the end of Mystery of the Emblem and has since been (partially) been seen again in various supplemental material such as the site for New Mystery of the Emblem and the deluxe art book for the western release of the first Fire Emblem game, Naga died at Thabes some 3100 years before the events of Fire Emblem Awakening.
Forneus mentions the following in the tablets he left behind: "At last, blood from a divine dragon! Its power is terrifying. Beautiful." He obtained the blood from somewhere, but it isn't specified from where. It seems likely then given the lore from Mystery of the Emblem and the weird connections between Naga and Grima in Awakening, that the Divine Dragon blood Forneus used to make Grima was in fact Naga's. Obtaining blood from a dead Divine Dragon would also be a lot easier than a live one.
Grima, who was once shrouded in mystery, is thus a being unique in the Fire Emblem series. He is a sort of homunculus: a horrible amalgamation of human and Divine Dragon blood, donated from Forneus and most likely Naga. And he looks pretty horrifying too with that big human face.