In the ancient Roman world (and indeed still to modern cultores Deorum Romanorum), the spirits of the dead were just as prevalent in everyday life as the gods themselves – if not more so. There are the lares, the protector-deities of one’s home; the lemures, the formless, wandering spirits of the dead who have not received a proper burial; the larvae, commonly associated with the lemures but also with the aspect of tormentors of the dead; the dii parentes, the spirits of one’s ancestors; and the dii Manes, the spirits of the dead in general.
As a collective, the dii Manes are the spirits of the deceased; as such, they are what is known as “chthonic” deities, or, in Latin, dii Inferi: “those who dwell below.” When a person died, they would often have a tombstone erected with the abbreviation D.M. upon it, which stood for diis Manibus. Curiously enough, early Christian tombstones also bore this abbreviation.
(In the convoluted grammatical system that is Latin, diis Manibus is in the dative case, which basically indicates that the deceased was now headed to or for the Manes, and would join their collective.)
There is speculation as to the origin of the word manes. The most popular theory is that it comes from manus, which is an archaic Latin adjective meaning “good.” If this is the case, then the dii Manes are to be seen as benevolent spirits.
St. Augustine of Hippo writes in his City of God (actually quoting an earlier author, Apuleius, the other of The Golden Ass [among other things]) that the lares are benevolent spirits of the deceased, the lemures and larvae are the malevolent spirits, and it is uncertain which group the Manes belong to. This is a set of beliefs which I subscribe to as well.
Long before the Empire (and even the Republic), the Manes were offered blood sacrifices in the form of gladiatorial games. At that time, the Manes were seen as rather bloodthirsty spirits, and so to appease them people would hold these “games” at funerals.
The Manes are honored in the Parentalia and Feralia festivals of February. The former is a festival dedicated to the dii Parentes, one’s ancestral spirits, and lasts for a good 9 days; the latter marked the end of the Parentalia and is celebrated on February 21st.
The lemures and/or the larvae have their own festival as well, the Lemuria. This is a really exciting festival, and one that is a household exorcism of sorts. Basically, the head of the household (the paterfamilias to the Romans) would get up at midnight and walk barefoot through his house, throwing black beans over his shoulder and chanting “With these beans I redeem me and mine” nine times. After this, the family would bang pots and pans together with a loud crash, reciting “Ghosts of my fathers and ancestors, be gone!” – also said nine times. The spirits were said to take the beans for themselves and be frightened away by the loud clashing noises.