This is the southernmost of the tors that line the impressive Brent or Chapel Hill, high above the town of South Brent. It is one of the largest outcrops on the hill, composed of metamorphic rock, but the nature of the location within dense woodland means that only portions can be viewed at any one time, and it is quite difficult to gain a full perspective of its size.
Marked in an open access area on OS maps, this doesn't seem to be the case as an impenetrable wire fence ascends the hillside from a rather broken gate below, where a bridleway enters the moorland from Lutton. The name of 'Underhill' might derive from the fact that the tor sits right down on the hillslope. The nearby farmhouse below is also given this name.
The Tithe Map for the Parish of South Brent notes this outcrop simply as 'Rocks' in plot 2164. A closer look indicates it is dissected by a wall, which is indeed the case as seen in the fourth photograph. Clearly seen from directly above but not from afar, this hidden tor must be the most secluded of the lesser-known rockpiles on this interesting hillside.