Black mass altars. Demonic images. All to be found in the cathedrals where people go for spiritual uplifting. The most evil goings-on depicted. Built by guilds of Freemasons. These are not Christian churches. Skulls & cross bones.
Evil spirits have to be called up in rituals using exactly the right name and words, or you get an imposter spirit appearing. The magicians had to be good at grammar. The rituals they used known as ‘grimoires’. The evil spirits they called up could appear as seductive figures such as a baby, a beautiful young woman or a sweet old person. Yet their real nature would be outright evil and they had other supremely ugly ways they appeared, either in the smoke of burning incense or other ways they appear during rituals.