Content warning for blood.

So I'm in the process of writing a novel. In general, the story is solidly in the PG-13 category in terms of sexuality and violence. However, the prologue contains some more graphic depictions of violence (The main character uses her blood to draw a magic circle while horrifically injured. Like, she's dragging blood through the snow every time she takes a step.). The character isn't human, if that matters.

I do plan on giving readers a trigger warning beforehand, but should I be marking that kind of content as mature, and does it matter if the prologue is marked mature when the rest of the story isn't?

Sorry if this question is silly, my comic has no mature content, so this is new for me.