Demonianism is a relatively uncommon term, but it is still important to have synonyms to convey a similar meaning. One such synonym could be demonolatry, which refers to the worship of demons. Another option could be demonism, which suggests the presence or influence of demons. A more general synonym could be the occult, referring to hidden or supernatural practices. Other related terms could include devil worship, black magic, or satanism. Ultimately, the choice of synonym will depend on the specific context and the tone that the speaker or writer wishes to convey.