The Pillar of Eliseg, originally an ambitious round-shafted cross, stands on a barrow near the Cistercian abbey of Valle Crucis. It was carved with a lengthy inscription, now illegible, but transcribed in 1696 by Edward Lhuyd. Two copies have survived, enabling a reconsideration of the significance of the inscription. This article reassesses the history of the monument, its archaeological context, form and function. The inscription shows that the cross was erected by Concenn, ruler of Powys (d ad 854), to honour his great-grandfather, Eliseg, who had expelled the Anglo-Saxons from this part of Powys. The inscription also links the rulers of Powys with the Roman usurper Magnus Maximus and the sub-Roman ruler Guarthigirn. It is argued that the inscription was intended to be read out loud and that the monument was an important piece of public propaganda erected at a time when the kingdom of Powys was severely under threat.