I would be an illusionist/magician, and the only boss in the entire game, and you would face me more than once. I would be a complete Psycopath, i would only want the world to feel pain. I would look and act as a more psycotic version of Mistgun from Fairy Tail
I will kill the Protagonists family, and injure him quite badly, as well as half of his country. After him training for a while and trying to get to me, I would be the first boss.
He would challeng eme, i would feel graceful and accept. His first few swings will pass right through me, and he would hear a laugh behind him, that one was an illusion. I would pull out a staff and block a few of his hits, Then i would jump back about 20 feet and hold my staff behind my back and point it upwards while holding my arm along it the entire way, and call out a name, the ground would warp, the roof would rip off and the Protagonist would look skyward, and see a massive monster come down from the sky, horribley angry and a fight would start between you and that monster. After the Protagonsit defeating the monster, he turns towards me and challenges me, not any monster, just me. I clap, while laughing, stop, look him in the eyes and say a firm "No", and would swing thr same staff towards him and try and cast a fireball towards him. The Staff would break. I would get worried and pull out a second one, slam it on the ground and in a puff of black smoke, would dissapear.
After finding and fighting me 4 more times, in varying ways, always against some massive monster, he would find me again, for the last time, i am down to one staff. I would look very angryly at him, and start yelling loudly, this black electric type of energy goes around me, and blasts the Protagonist back, i pull out my last staff and slam it on the ground, the floor opens up similar to an earthquake, and a massive devil looking creature appears from there, and i am up on his shoulder. This last fight you have to fight th devil and me standing on top of him casting healing magic on the devil and destruction magic at you. After finnaly cutting down the devil and just left with me, my last staff breaks. The Protagonist advances at me, I fall onto my knees, he goes for the killing blow which suddenley stops an inch from my neck. A blue sheild starts glowing, and expanding. I stand up and laugh, maniacly, and say, "This is my Nuclear option, your f***ed, you and everyone else, good f***ing luck" and pass out, collapse on the ground. While the blue sheild is growing, the Protagonsit tries to stop it, and fails all the time. After almost giving up hope, he would look out the window and see a beautiful field, and he imagines it getting destroyed, power surges through him and he charges into the shield, peircing it, and making it explode and destroying the entire castle were the last fight takes place. Killing me in the explosion and mortally wounding himself, townspeople go into the castle and see a bleeding me and the Protagonist next to each other, they all crowd around him and start singing hyms in his nameand celebrating his name.
For all those idiots who tl;dr, I would be a magician illusionist who would want the world to die. And be the only real boss in the game...