The Horsingdon Transmissions No.76: The Thing Below Hallowmere.

This slab sits on a piece of waste ground around the back of Hallowmere Playing Fields, where one of James Boreham's property's once stood. Apparently the house itself collapsed in the aftermath of a particularly sinister episode involving Boreham's esoteric investigations; decades after Boreham's death (or disappearance, depending on who one listens to), the plot on which it was built was fenced-off by representatives of Ministry (citing vague concerns regarding public safety), such that it has not been accessible since the mid-1960s.

The above photograph was taken at a difficult angle (and slightly out-of-focus) through a break in the iron railing which surrounds the land, and thus fails to depict the sigils supposedly etched across the surface of the slab - which is apparently situated where a neolithic barrow once stood. Boreham bought the land and built upon it with the intention of excavating the barrow, hoping to call up something which, according to certain ancient and diabolic texts in his possession, had long slumbered in some benighted cavern deep below the Hallowmere.

The slab as it exists today was once part of a large granite rock formation which had originally covered the entrance to the barrow; Boreham had it carved - following arcane geometric principles drawn from his store of occult tomes - into a very specific configuration by local stone masons, who were then instructed to inscribe glyphs of warding across its surface. In his quest for forbidden knowledge, it seems that Boreham had indeed succeeded in calling forth something from the subterreanean depths below Horsingdon - something which not only laid waste to the property he had built on the site, but against which Boreham found himself powerless in his attempts to cast it back into the Outer Dark from whence it came; his only recourse, it seems, was to employ what praeternatural knowledge he did possess to seal off the portal to whatever lightless realm from which the thing had originally crawled forth - presumably in the hope of trapping it there permanently...