... It's because he is immortal. He won't get pwned by getting a huge hole in his stomache, after getting his head chopped off and surviving it. If the Ritual Circle was fake or it was a genjutsu or something like that, he will simply say "Damn, I pierced myself for nothing, dammit" and will not bother about it. Cuz he's immortal. Immortal people can survive getting their heads chopped off, their stomach being pierced. Plus, if the one who took the damage was him, he'd have been long dead, trust me, that styche like thing is much more deadly than a simple spear, when both of them pierces through your stomach. He couldn't have stood and pierced himself like that he was the one who took damage. Yup.