I was one of the people who played the original Onimusha of the PS 2 version and after almost 18 years, the remastering is still great as i remember it. Now i have decided to speedrun with this game. Before doing so, I would like to ask you about the Ultimate Weapon that's in Demon World, as i still remember, with the weapon it's allowed to defeat Fortinbras, is it allowed in the run? I know currently the remaster, the run time is in IGT (I still remember the results after the credits)

Yes it's allowed, because it's part of the game mechanic. I don't see the reason why it wouldn't be allowed. Although, you are still going to prioritise magic first as those are actually more effective to knock down / dps Fortinbras.