The middle cairn in a group of three cairns located along a track on Mynydd Epynt (NPRNs 305014 and 305016, a further possible cairn site (NPRN 90076) lies c.235m south-west of Cairn III.). A 1934 excavation of one of the cairns by Col. John Lloyd, Hon. Sec. of the Breconshire Society found a layer of ash and black soil, as well as kerb stones, but no central burial. This led Lloyd to conclude that the cairns had originally been waymarkers for drovers.

This grass-covered cairn is oval in shape measuring roughly 22.3 m. long (north-east to south-west) and 17.1m wide and between 1.8 and 2.1m high. There are traces of a trench near the centre. In 1989 Cadw described two peristalithic stones on the southern side of the cairn, but this is contradicted both by descriptions by the RCAHWM (1997) and CPAT (2004) who describe these stones as situated on the south-eastern and north-western cairns, respectively.

Brynley F. Roberts has suggested that the Tri Chrugiau may be the cairns referred to in Walter Map's De Nugis Curialium (Distinctio ii: xi. `De aparicionibus fantasticis?), in which King Brychan of Brycheiniog supposedly deposited the right hands, `virile members? (mentulas eorum), and right feet of the defeated army of Deheubarth, which had been led by Triunein Vagelauc. Although Walter Map writes that the cairns were named after the body parts buried therein, these names do not seem to have survived.

(Sources: NMR Site File, Breckonshire/Earthworks/SN94SW; Walter Map, De Nugis Curialium, ed. by James, revised by Brooke and Mynors (1983), pp. 149?55 (esp. pp. 152?5); RCAHMW, An Inventory of the Ancient Monuments in Brecknock (Brycheiniog): The Prehistoric and Roman Monuments, Part i (1997), p. 104; Roberts, `Melusina: Medieval Welsh and English Analogues?, in Boivin and MacCana (eds), Melusines Continentales et Insulaires (1999), pp. 281?95 (esp. pp. 290?91); CPAT Regional Historic Environment Record, 2004)
A.N. Coward, RCAHMW, 03.07.2018