The Black Abbey or the Convent of the One and Undivided Trinity was founded by William Marshall the younger, Earl of Pembroke, for the Dominican Friars in 1225.

The site was an unfortunate one as subsequent events proved, for to this day, it is subjected almost annually to severe flooding. Despite this handicap the Abbey has shown vitality and tenacity of life rare in monastic institutions.

From its earliest days it occupied a place of importance in the civic life of the community. The 'Great Twelve', the governing body of the early Norman town, frequently met there, and the lords Justices stayed there when they came to carry out their legal functions.

The present building includes the restored nave and ancient tower house, which serves as an organ gallery, together with the large early fourteenth century south transept and magnificent window with its fine intersected tracery. The high, square bell tower with its attractive Irish-style stepped battlements was built around 1505.

An unusual sculpture in alabaster of the Blessed Trinity may be seen near the high altar. This was found hidden in a wall of the ruin during the 19th century restoration and bears the inscription, in arabic numerals, 1264.