Do We Have This
? A character (usually a king) fancies himself a God, or at least immortal, even though he isn't; he was tricked into thinking so by someone else (usually the evil vizier or similar) or perhaps just thinks he's immortal because... well, he's crazy.
As soon as The Hero
arrives to fight him (kings that think themselves immortal are invariably evil), and manages to scratch his face, or his chest, or his leg, and a single tiny drop of blood falls down.
And then the man who fancied himself immortal FLIPS THE **** OUT, calling for a medic or the guards or trying to stop the tiny streak of blood with his own hands.
So yeah, a trope about people who realise they bleed, and freak out because the idea of vulnerability never occurred to them before.
I've seen it a couple of times. Definetely enough to make a trope out of it. One was in Conan the Barbarian comics, I think, but I can't remember the name of the king in question...