The oldest ghost dream in European literature is in the Iliad. The night after Patroclus dies, his shade comes to the sleeping Achilles, exact in likeness down to the voice and eyes, and asks for one thing: burial, quickly, so that he may pass the gates of Hades where the other shades now bar his way. Achilles reaches for him and closes on smoke. The scene fixed the pattern the traditions kept for millennia, the dead returning in sleep not to terrify but to require something, rites owed, wrongs unsettled, remembrance kept.
The pattern is not only literary. Roman and Chinese practice alike took dream visits from the dead as claims, ancestors requiring offerings, the unburied requiring graves, and the medieval church catalogued dreams of souls asking prayers. The obligation reading, the ghost as unfinished business in the most literal sense, is among the most widespread dream conventions on record.
Modern bereavement research renamed the experience without dissolving it: visitation dreams, in which the dead appear alive, well and usually at peace, are common in grief, characteristically vivid, and are reported by the bereaved as consoling rather than frightening. The psychological traditions read the haunting kind of ghost dream toward the past that has not been laid, memory or guilt still walking; the visitation kind they mostly leave to do its evident work. Both readings are the traditions' own, and Homer, on the evidence, already knew both.