One of the 'rogue' stones connected with the henge site of Stonehenge on the Salisbury plain in Wiltshire, UK.
Stonehenge has often been used as, or described to be a Druidic ritual site, a Celtic temple, a UFO calling centre, and a centre for paganism and witchcraft, as well as the more likely explanation of a landmark meeting place for a Mesolithic and Neolithic hunter-gatherer society. The Slaughter Stone is part of the pagan legend that tells of the site being used for ritual sacrifices, virgins being chained to rocks and slaughtered to welcome in the coming of the Summer Solstice or Midwinter sunset. (The site is proven to line up to these astronomical alignments, the entrance lining up with the Midsummer sunrise, the 'Great Trilithon' appearing to be a frame for the sunset on the day of the Winter Solstice.)
The slaughter stone lies outside the main henge, to the left of the entrance, and appears to be streaked with the blood of various sacrificial victims from these days of gore.
However...
There is no evidence that the stone was used as a sacrificial tablet. For one thing it would originally have been an upright stone, rather than lying on its side as it is today. Also the 'blood stains' are a natural phenomenon caused by iron ore within the natural matrix of the sarsen rock. There are no human sacrifices anywhere around Stonehenge, which has been a site of intensive archaeological investigation for the past century. At least, there are no human remains which have yet come to light which are connected to the site and ritual practises. The only burial in the area is that of an archer, pierced quite convincingly by a large amount of arrows, that lies within the henge bank.
The Slaughter Stone is a misnomer, a name associated with the mythos of a site which has plagued academics as to its origins for hundreds of years. Despite rigorous excavation Stonehenge, like most other prehistoric sites, is keeping the secrets of its past to itself. With only a paucity of environmental evidence to draw conclusions from, the true nature of these sites will continue to be a mystery open to wild and spurious conjecture, until someone finally invents the time machine. Just hope I'm around to see it!