The Dweller on the Threshold refers to an invisible malevolent entity that attaches to a human being. The term was first used by Bulwer-Lytton in his novel Zanoni.
One explanation is that provided by Alice Bailey on Esoteric astrology, "From ancient recesses of the memory, from a deeply rooted past, which is definitely recalled, and from the racial and the individual subconscious (or founded and established thought reservoirs and desires, inherited and inherent) there emerges from individual past lives and experience, that which is the sumtotal of all instinctual tendencies, of all inherited glamours, and of all phases of wrong mental attitudes; to these, (as they constitute a blended whole) we give the name of the Dweller on the Threshold. This Dweller is the sumtotal of all the personality characteristics which have remained unconquered and unsubtle, and which must be finally overcome before initiation can be taken. Each life sees some progress made; some personality defects straightened out, and some real advance effected. But the unconquered residue, and the ancient liabilities are numerous, and excessively potent, and - when the soul contact's adequately established - there eventuates a life wherein the highly developed and powerful personality becomes, in itself, the Dweller on the Threshold.