Demon: Phantom
Requirements: Surt (any variation)
Head to Souhonzan and find Yoshikuni the Gaian. Talk to him with Surt summoned and he'll give you a quest to kill the Balance-Threatening Phantom in Nakano. Phantom spawns slightly to the south of the Yagiya during the New Moon. Once you kill him, report back to Yoshikuni, and he'll give you the Phantom Plugin: Death's Shadow as a reward for your services.
The Blessed Light