The Demon's Captive Content Warnings

Kinks

Graphic sex
Dubious Consent
Bondage
Primal play
Exhibitionism
Praise/Degradation
Mind Control on her
Kidnapping (MMC captures FMC)
Punishments (whipping, spanking, edging)
She wears a collar that keeps her in his castle
BDSM (Dominant yet caring MMC, bratty sub FMC)

Other Themes

Murder (not of a main character)
Mentions of rape (not between the main couple)
Mentions of pregnancy and contraception
Attempted non consensual sex acts (not by MMC)