Henshin!! Kamen Rider Wars

A science organization has created a portal to hell in order to study demons and unlock the secrets of immortality. They have created the rider system, a belt that allows a human to fuse with a Contract Beast to gain greatly enhanced powers provided they are compatible. But they got over their heads and now the demons have escaped causing havoc amongst the populace. You are amongst one of those who are lucky or unlucky to be able the rider system and it is now your responsibility to fight those demons and save humanity. Or you can just rule them, you have the power anyway.