Fate/ A Demon Promise

Five months after the defeat of the dark triad and the king of devils, Luifero. Asta, the Anti-Magic Knight, while training found a cave in the forest near his hideout. Going into the cave he found a portal and after an incident happened the portal goes haywire, sucking him in it and sending him to another world much different then his own where a silent war was happening, now he has to find a way to win the war and to get back home. Will he be able to do it, or will his spirit be broken in the face of absolute destruction?

(I don't owned the rights to the character in this story. both of these series are own by their respective creators.)