Doctor Who - The Shadows of Avalon - Paul Cornell

Centuries ago, Jasper Kain transformed into an immortal warrior when an ominous phantom known as the Shadow King seared mysterious circular runic designs into his palms, a curse with forbidden blood magic.
Demonic armies invade Japan and rampage through the streets of Tokyo, demolishing everything in sight. Vincent Angelo, an unhinged stranger appears, bearing the same cursed tattoos as Jasper, and answers to a dark prophecy foretelling the end of the world; hidden within the shadows of his fractured mind.
Jasper, together, with fellow Chosen Knights, Hanako, and Tenchi, face many perils while fighting against powerful mythological adversaries. The heroes cross realms and risk everything to awaken the Children of the Gods; The Holy Mages, who are sworn to protect the heart of magic, Avalon.