The information board gives two legends associated with this destroyed tumulus.
The first, and the source of the "Sacrifice" tag, suggests that the straight groove formed along the middle of the stone was used "aux sacrifices rituels et a la magic noire", (for sacrifice rituals to do with black magic). Presumably the "victim" was sacrificed upon the stone and the blood would be collected as it dripped down the groove into a cup or chalice. This would point to the stone being in a more or less flat position when the tumulus was intact, possibly forming part of the capstone or roof.
The groove is hardly natural and does not travel the whole width of the stone. See enclosed photos. There was record of a Sheppard's crook or Crosse being carved into one of the upright stones too. Whether this was ancient or done in the later Christian era is unclear, as is also if it survived the 19th century destruction. Maybe it was carved by the church authorities in order to "sanctify and purify" a pagan relic.
A much more ancient legend tells of the "petits hommes", the little people who were said to inhabit the forest. They were said to have built the tumulus and made it there home. They were so strong that they could carry the enormous 15 tonne blocks with their bare hands. Maybe they were the faries and they could move the huge stones using their magic.