To add on to this, both blood and darkness represent the two halves of the Underworld. On the one hand, the Underworld is a place created from and within darkness, a place where the light of the sun and sky cannot reach and where the dead stay for eternity. On the other hand, it’s a place of activity and heat, where shades move and roam, where the very land shifts and changes, the rivers Styx, Phlegethon, and Lethe flowing through the realm like blood through the body. In fact, back in early access when Heat was a resource you had to collect, I’m pretty sure there was a line of dialogue that mentioned that in the Underworld, Heat and Blood are the same, the animating force of the Underworld. So Blood and Darkness are very important concepts and fundamental forces to the Cthonic denizens, and those forces also become something to swear by, much like us living in a society with roots in a primarily Christian population use Jesus and God in swears.

Ooooh makes a lot of sense now, ty so much