I suspect that Shaon was Toa's father. Toa said that her father was able to defeat a Leowolf, an S ranked monster, all by himself. Something that would require knights from all around the kingdom to band together, to take down. So we know that Toa's father is unusually strong. Shaon was unusually strong. When Masamune fought Shaon, he made a comment about it being 150 years since the last time an adventurer tried to kill him. Shaon was sitting on a throne in what seemed like a dungeon, but could have easily been a castle. It's more likely a castle because there was a storage room filled with wine and maybe food also.
We know that Toa is a demon and that she grew up in a castle. The most logical conclusion is that Toa's father was the stereotypical Demon Lord, and probably also the one the King who did the hero summoning, is fighting a war against. It's the standard trope of having a girl fall in love with the main character; only to discover later that he killed her father before they even met. Given how generic and predictable most of the writing is, I'm probably right on this. But I have not read the light novels. Nor have I read ahead in the manga, as of the time of me writing this. No spoilers please.