The Dweller on the Threshold refers to a purported invisible and possibly malevolent entity that attaches to a human being. The term was first used by Bulwer-Lytton in his novel Zanoni.
In theosophical literature, Helena Blavatsky describes it as the discarded astral double of an individual in a previous life that may not have fully disintegrated yet when that individual is reborn. Thus the dweller will be drawn to the new incarnated personality due to their affinity. Sometimes this entity is also called Guardian of the Threshold.
Another 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. Then the Angel of the Presence and the Dweller stand face to face, and something must then be done. Eventually, the light of the personal self fades out and wanes in the blaze of glory which emanates from the Angel. Then the greater glory obliterates the lesser. This is, however, only possible when the personality eagerly enters into this relation with the Angel, recognises itself as the Dweller, and - as a disciple - begins the battle between the pairs of opposites, and enters into the tests of Scorpio. These tests and trials are ever self-initiated; the disciple puts himself into the positive or conditioning environment wherein the trials and the discipline are unavoidable and inevitable. When the mind has reached a relatively high stage of development, the memory aspect is evoked in a new and conscious manner, and then every latent predisposition, every racial and national instinct, every unconquered situation, and every controlling fault, rises to the surface of consciousness, and then - the fight is on."