The north-eastern most of a group of three cairns located along a track on Mynydd Epynt (NPRNs 305015 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 circular but slightly truncated by a track on the west, with a trench running across the centre of its top. It measures roughly 20m in diameter and 1.8 metres high. In the 1997 Inventory of Breconshire, the RCAHMW identified rig and furrow abutting on the west, although in their 2004 report CPAT has suggested that this `may be vehicle tracks, though they are suspiciously regularly set?. According to CPAT, there are also two large peristalithic stones, one over 0.6m in diameter and the other around 0.3m in diameter, at the southern base of the mound (described by RCAHMW (1997) as being on the south-west cairn and by Cadw (1989) as being on the central cairn).

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