In the Cradle series, some few Blood Shadows are actual characters, even though they are blood spirits. The Sage of Red Faith first researched methodology for utilizing Blood Shadows to gain power and advance.
The Bleeding Phoenix is the only Dreadgod that can be physically destroyed. However, it's only a temporary defeat, because the embattled bird molts thousands of its feathers as "Blood Shadows" that scatter far and wide. Each Blood Shadow carries a fragment of the Dreadgod's power and consciousness.
When a Blood Shadow finds a powerful sacred artist, it persuades the artist to allow it to nest in their core (like an egg). Then it parasitically begins feeding from the host's madra, blood, and life force. The Blood Shadow returns to the Bleeding Phoenix when it is full to deliver the stolen power. In this way, the Bleeding Phoenix gets stronger as it rests. This is the basis of power used by the parasitic followers of Redmoon Hall, one of the Dreadgod Cults.
Mu Enkai and the Egg
A Blood Shadow tale...
Mu Enkai had been nothing before the egg. He was born a servant, following just enough of a fire Path to allow him to operate a furnace for a low-ranked refinery. He’d spent his days shoveling coals and choking in smoke, making barely enough scales to live on. Then the sky had turned red. He and his fellow servants had taken shelter in the cold furnace, huddling together for days before the earth stopped shaking and the air was no longer flooded with blood aura. When they emerged from hiding, shaking but alive, it had been waiting for him. The egg was a glossy, polished orb the size of a man’s head, waiting among the debris of the refiner’s shop. Unlike the building, it was unharmed. Spotless. Beautiful. He could feel its promise in his soul; the blood aura that had drowned the world for days was concentrated here. Enkai had never considered himself an ambitious man, but in the egg’s sleek shell, he’d seen his future. (Ghostwater prologue)
Known Shadows / Hosts
Longhook -- his Blood Shadow is / was his long bloody hook (see book 5, Ghostwater)
Red Faith, the founder of Redmoon Hall, has a Blood Shadow. He fed it his life energy and his own blood to control it, like a carrot, but it isn't truly loyal. So, it escaped his control and independently advanced to Herald. It now sits like a king, and dubs itself the Herald of Redmoon Hall. Or just Redmoon. (see book 10, and see Sage of Red Faith
Yerin's Blood Shadow
As far as we know, the only sacred artist to resist a Blood Shadow — which is to say, a Dreadgod -— is Yerin, when she was only a child of seven. For almost ten years, she wore that Blood Shadow as a belt. Eventually, she began to utilize her unwelcome guest. This occurred first at the end of book 4, Skysworn, when the Bleeding Phoenix had passed overhead and her Shadow was trying to escape and attack.
- She was guided in how to use her Blood Shadow by Eithan Arelius and the dream tablets he stole from the Sage of Red Faith, aka The Blood Sage.
- In book 8 Wintersteel, her shadow adopted the name Ruby. Ruby appears to have a thing for Wei Shi Lindon.
- Eventually, Yerin merged with a willing Ruby, and her power grew enormously. She has blood madra now. And she can employ a huge bloody Netherclaw weapon now. And now she is a Herald. See book 8, Wintersteel.
- The Sage of Red Faith is obsessed with learning how she merged with Ruby and yet remained in charge. He's obsessed with Yerin, too. Stalker crazy.
Bloodspawn are nasty little agents of the Bleeding Phoenix. Within the Phoenix's miles-wide range of blood aura, natural spirits known as bloodspawn are formed. Vaguely humanoid in appearance, they rise from sources of blood, including open cuts on still-living people. (Skysworn, Chapter 9; Bloodline, Chapter 15) They aim to wound those around them, increasing their own numbers and providing more blood madra for Phoenix, their master. Blood Shadows appear to be able to control bloodspawn. At least, Yerin could stop them, with her Blood Shadow, at the end of book 4, Skysworn.