Takuma Sakamoto, better known as Diablo, has been through quite a bit as of late. After becoming a king, he found himself unable to avoid the attention of the king of Lyferia, Delouche Xandros. With his strength known, the people in power looked to use his strength for their own agenda, and so they sent him to the south to deal with a problem.
Kobolds are a race of "monsters" who very narrowly missed the mark to be considered demi-humans. In the past, it was determined that the ability to speak the common language, to be able to communicate, was what made a species fundamentally Human. Kobolds could not learn the common tongue before the distinction was made, and so they were to remain persecuted.
Diablo, as a side effect of being able to understand the Lyferian language, one he is not natively able to speak, is able to understand any language he hears. With this ability, he is able to understand the kobold language, calling into question the judgement that they're not people.
He was also brought to question the ethicacy of destroying this group of people. In trying to change the minds of the people leading the charge, he became their enemy, and was forced to fight one of his companions last remaining family members, attaining a narrow victory after a harsh battle.
This was not the only or last conflict of the night, but this "betrayal" of Lyferia turned Diablo into a traitor. As a king, and an immensely powerful sorcerer, Lyferia could not ignore his actions, and because of his allies in the capital, he couldn't simply run away to his kingdom. Deciding to take the long journey back to the capital by wagon, Diablo missed all news of the encroaching army.
The Gelmud empire hails from the far east. They utilize a strange and unique form of magical science, one that gives the ordinary soldier an edge in the form of modern weaponry. That is, rifles and machine guns. There are also a set of elite soldiers that more than rival Lyferia's top hitters, who are usually sent as a vanguard force. They pilot massive constructs, called Magimatic Sols, that meld bio and mechanical aspects to great effect. Standing at an average height of 15 feet, they are a menacing force. Quickly advancing through eastern Lyferia, the Empire only needed to win a few key battles to deeply de-stabilize the Lyferain government.
This essentially exonerated Diablo, as his lone presence was suddenly much less important than the looming subjugation. Even the demon lord Krebskulm, said to be the strongest of all demon lords, felt a need to directly combat the looming threat of the Empire on behalf of humanity. Diablo, as much as he would have liked to run away and hide, had a few things pushing him to defend against the enemy. His friends and allies couldn't just leave like he could, and he detests needless murder.
And so, Diablo made his stand. As late as he was, he made it in time to end the first conflict. Fighting the Sols was difficult, but not more so than some of the fights he had already been in. In his first fight, he spent much time going back and forth, playing safely, which wasn't innately a bad idea, but in taking so long, he endangered those he held closest. As it turns out, Rem is the person the empire wants to capture because of the unique nature of her body. After holding the soul of a demon lord for so long, she is uniquely able to house "large" or powerful souls within her, and to maintain them by simply remaining alive.
Using some great, unknown magic, the emperor was able to identify rem, or who he refers to as "the girl of vessel", and announce her presence to the battlefield. Sols in other battlefields notice this as quickly as Diablo does, putting him on a timer. Diablo finally gets serious, using large, aggressive spells tactically to kill the last Sol pilot left to deal with him.
Rem isn't weak, but the Sols exist on another level. She gets brutalized, used as a scapegoat for all the pilot's resentment at their mistreatment by the empire. Diablo gets there before she is killed and saves her with powerful potions, but only barely.
After this, they regroup in the Inner Sanctum. More allies show up, and they plan on making an attack on the castle, after retrieving equipment from Diablo's dungeon. The attack proceeds as planned mostly, with the help of secret routes making it quick. There are a number of sols defending the enemy general, but Diablo's group is strong enough to force their way through. Diablo kills Doriadanph, the general of the enemy army.
Soon after, a spectre of Emperor Gelmud appears in the sky and tries to abduct Rem, but Diablo dispatches it with some difficulty. This spurs the Royal Knights, led by Noah Gibun, to make a move on Castle Gelmud, a massive fortress carried by massive mechanical legs over land. Krebsulm, known as Klem, is an actual demon lord, pacified by diablo early in the series. She shows up unexpectedly with Noah, and pleads for Diablo to assist the raid. He eventually agrees after the Emperor casts a spell so large and destructive it removes an entire district of the capital.
The raid goes poorly. They teleport to the surface, but are immediately stopped by a captured and mind controlled ally of Diablo. Leaving two members behind, the rest venture into the castle, led by a freed enemy commander. Members of the group are slowly picked off one by one, Diablo and Klem are separated from the group, and those remaining make a futile final attempt to attack the Emperor.
Diablo and Klem discover the corpse of god, desecrated by the Emperor, as well as a weapon that could defeat him. Refusing to use it to maintain his image, Diablo takes on the power of Krebskulm and fights the Emperor, saving just Noah in the process. After a difficult fight, Diablo wins with help from the freed enemy commander and his companions. However, in the Emperor's last moments, he curses Diablo to death, dragging him to the interstice between worlds. Klem is brought with as well, as she is effectively an extension of Diablo as he "dies".
The passage of time is difficult to parse where they are, so he passes it by learning the intricate workings of magic from Klem. After an unknown amount of time, maybe days, maybe months, maybe years, Diablo receives a call from another world. He is being summoned. Against Klem arguing that it could fail, he grabs her, bringing her to the new world with him, modifying and assisting the spell to be successful with both of them.
However, he has no clue where he is, and neither does Klem. His mind racing, he wonders how they can make it out of this one.