This is the term for “Stone of the Underworld,” or of the dead, a sacred stone that covers the pit of the manes on the Palatine Hill in Rome. During the annual festival of Mania, the Ancestral Moon-mother, the stone was removed and her children the manes or ghosts-of-ancestors were invited to join the feast. Occasionally the festival was called Parentalia, since it was believed the ghosts of the underworld were synonymous with the di parentes or “parent-gods” from past ages.
In Northern Europe similar festivals were held at Halloween or Samhhain (see Sabbats), when ancestral ghosts were invited. A.G.H.
Source: 56, 529.