They have 50 HP.
Only appears when summoned by a Cultist.
Its shield heavily reduces all frontal physical damage. After coming close enough, it will swing its sword for 12-15 damage. You should try to bait out its attack, jump over it and attack it from behind. Alternatively, spells and falling attacks such as the Knight's Omnistab or the Wildling's Brawler affinity are good ways to deal with the Demon Armor.