Well, when you think about it, his boss fight is pretty weird in that regard
He doesn't say a single thing throughout the entire fight, doesn't even grunt when hit, doesn't bleed and his movements are stiff, almost robotic in nature. He's supposed to be an absolute master of Golden Order Fundamentalism (to the point he personnally created at least one Incantation) and is stated to have mastered Glintstone Sorcery yet doesn't use either of them, instead he uses golden light/energy that don't ressemble any Incantation or any kind of spell since he doesn't use complex, ritualistic gestures but feels more like raw energy overflowing him (to the point he can do things unrelated to fighting moves and still passively release blasts and waves of energy), like if it was animating him.
We also see his body emitting shadows from which the Elden Beast emerges after the boss fight against. Adding to the fact that we know the Beast is the who tortured and mutilated Marika (and thus Radagon since they share a body) since one of its moves invoàlevs crucifying the Tarnished on an Elden Ring arc and then impaling them with an "elden spear" like Marika, the fact that Radagon's body is so broken it's literally empty (as in, there's no organ where they should have been) and is filled with the Elden Ring before the fight and the three golden projectiles he creates looks like the projectiles used by the Beast...