The question of Bernard's participation in the Battle of Hastings and therefore in the Norman Invasion is subject to debate. While Bernard had close family connexions to the port of Saint-Valery-sur-Somme from which William's invading fleet launched, Bernard himself was not the ruler of that city and need not have been in the fleet. He had later connexions with Battle Abbey: he established a cell of that abbey in Brecon, but that may have been an analogous foundation intended to mark his conquest of Brycheiniog. Bernard's peculiar absence from the Domesday Book more or less damns the case for his presence at Hastings, for it is impossible that a noble participant in the victorious battle should not have received land to be recorded in Domesday if he was still living in 1087.
Probably as a consequence of his rapid rise in the marches, Bernard attracted the attention of Osbern fitz Richard, who gave him his daughter, Agnes, whose mother was the Welsh princess Nest, daughter of Gruffydd ap Llywelyn, in marriage sometime before 1099. She brought with her a dowry of Berrington and Little Hereford.
All of Bernard's estates lay in the valley of the river Wye and along an old Roman road which led from Watling Street to Y Gaer and on into Brycheiniog. The military possibilities of that road could only have encouraged his subsequent ventures into Wales.
The chronology of events at this juncture is often confused. Bernard may well have already been in power in Brycheiniog by 1088 if he had already inherited a claim to it after the defeat of Roger de Breteuil, Earl of Hereford, in 1075. In 1088 the king, William Rufus, confirmed a previous charter of Bernard's stating that he had already made an exchange "within his lordship of Brycheiniog" at Glasbury. He also already held Castell Dinas which had probably been built by the Earl of Hereford before 1075.
After the initial conquest of 1088, Bernard continued warring with Brycheiniog unti 1090, probably supported by Richard fitz Pons, the lord of Clifford. Talgarth was captured early and a castle was constructed at Bronllys where the rivers Dulais and Llyfni meet, a site probably central to the llys of the tywysog of the commote of Bronllys. By 1091 Bernard had reached the valley of the Usk, which was at the centre of the kingdom which was to become his own principality.
There is some discrepancy in this description of events also. Richard Fitz Pons was lord of Llandovery, which he had reached probably through Glamorgan, already by 1088. Bronllys Castle may not have been built until 1144, when Roger Fitzmiles, Earl of Hereford, is first recorded granting it as a five knights' fee mesne barony to Walter de Clifford, son of Richard Fitz Pons.
According to much later accounts and reconstructions, the accuracy of which is very dubious but which contain some references to verifiable history, the king of Brycheiniog, Bleddyn ap Maenarch, allied with the king of Deheubarth, Rhys ap Tewdwr, in 1093 (or perhaps 1094) and tried to attack the forces of Bernard which were building a castle at Brecon on the Usk and Honddu in the centre of a great plain in his kingdom where several Roman viae met. Bleddyn led a charge up the hill, but the Normans defeated the Welsh and Rhys was killed in battle. Brecknock Priory, which was later founded at the site of the battle, may have been built on the spot where Rhys supposedly fell. Bleddyn died not long after and Bernard was able to advance over the whole of Brycheiniog.
Reliable historical records refer to no king of Brycheiniog after a Tewdwr ab Elise who died after 934. Certainly there is no contemporary reference to a Bleddyn ap Maenarch. The Welsh Bruts simply state that "Rhys ap Tewdwr, king of Deheubarth, was slain by the Frenchmen who were inhabiting Brycheiniog." This passage lends evidence to the belief that the conquest of Brycheiniog was mostly finished by Eastertide 1093 and that the main effect of the battle of Brecon was to open the way to the conquest of Deheubarth.
Two expeditions from Glamorgan came to the rescue of the garrisons of Brycheiniog. The first was crushed in battle at Celli Carnant, but the second defeated the rebels at Aber Llech. What followed was the complete encastellation of Brycheiniog. Among the castles possibly built during Bernard's lordship to defend the entrances to Brycheiniog from the southeast were Tretower, Blaen Llyfni (not attested before 1207–1215), and Crickhowell.
Bernard also extensively enfeoffed his followers with Welsh land. Richard fitz Pons may have been enfeoffed at Cantref Selyff on the western border of Brycheiniog and immediately he began in miniature the process whereby Bernard had come to rule Brycheiniog. However, Richard's son Walter is the first recorded landholder at Cantref Selyff. Furthermore, Bernard enfeoffed the sons of the king he had displaced in the less habitable land, thereby creating a loyal Welsh aristocracy and extracting more out of his land than the Normans otherwise knew how to do. The Normans lived predominantly in the valleys and lowlands in an agrarian society while the Welsh kept to the hills and mountains living pastorally, thus creating an overall economic gain. Among Bleddyn's sons, Gwrgan received Blaen Llyfni and Aberllyfni while Caradog received an unnamed hill country, and Drymbenog, Bleddyn's brother, was given land neighbouring that of Richard fitz Pons.