In the Battle of Mauriacum (formerly referred to in error as Catalaunum), 'also present, at the head of an immense army of Gepids, was Ardaric, the most famous king, who, thanks to his loyalty to Attila, could participate in the latter's councils. Attila, who possessed sharp judgement, preferred him to the other vassal kings; Ardaric had earned renown for his loyalty and sound counsel'.[9]9. 'eratque et Gepidarum agmini innumerabili rex ille famosissimus Ardaricus, qui ob nimiam suam fidelitatem erga Attila eius consiliis intererat. Nam perpendens Attila sagacitate sua, super ceteros regulos diligebat; Ardaricus fide et consilio clarus' (Jordanes, Getica, 199). Jordanes drew for this characterization from Cassiodorus' Gothic history, which was based in turn on data furnished by Priskos, who lived at the time of these events; to be sure, the Goths' historiographers had clumsily interpolated that the Ostrogothic King Valamir was also Attila's most 'preferred' vassal.

There is no doubt that, in 451, Ardaric — together with his Gepids — already stood 'at the Lord's right hand'. Their rise stemmed from their very defeat. The Huns had defeated and killed the royal family, whose authority had been shared with the tribal chiefs' council, as well as the most of the nobility, but they allowed ordinary Gepids to remain on their land in the valleys of the Upper Tisza, Bodrog, Ér, Kraszna, and Szamos. When the Huns' centre of power shifted to the Hungarian Plain, the Gepids were left as the sole significant compact group within the Hun sphere east of the Danube. With no other potential allies in the vicinity, the dominant Huns soon came to rely economically, politically, and militarily on the Gepids. The new king imposed on the Gepids, Ardaric, did not become the 'chief administrator' of a clan-based society, or the 'chairman' of the tribal chiefs' council, but a ruler whose power over his people was as absolute as that exercised by Attila over the notables and general population of his empire. This unlimited power was based neither on election nor on ascendance; it was simply {1-186.} bestowed upon Ardaric — and upon other vassal monarchs — by Attila and his Huns. The clever Ardaric was one of those who turned this power to their own people's advantage, a fact confirmed by the Gepidic finds from the Hun period.

When Attila died unexpectedly in 453, Ardaric and his army, which had gained experience in the campaigns against the two Roman empires, showed themselves to be masters of the situation. Ardaric saw his power imperilled by Attila's sons, who vied for the succession and fought for shares of the father's domain. Ardaric was the only vassal with the means to challenge Attila's son Ellák, and he drew in his wake the Svebs and the Rugians from the upper reaches of the Danube, as well as Edika's Skiris. The struggle was waged mainly by the 'furious sword-wielding Gepids (ense furens Gepida), and in the battle by the Nedao River, in 455, it was 'Ardaric's sword' (Ardarici gladius) that earned a decisive victory.

Following their victory, 'the Gepids forcibly occupied the Huns' land and conquered the entire Dacian territory. From this position of strength, they issued a demand to the Eastern Roman empire for no more than a friendly pact (of alliance), peace, and an annuity'.[10]10. 'Nam Gepidi Hunnorum sibi sedes viribus vindicantes totius Daciae fines velut victores potiti nihil aliud a Romano imperio, nisi pacem et annua solemnia, ut strenui viri, amica pactione postulaverunt' (Jordanes, Getica, 264); since the event was relevant to Eastern Rome, Jordanes probably drew the information from Priskos. The demands were granted, and, apart from two troubled decades in the middle of the 6th century, the Gepids served as external allies of the Eastern Roman empire.

Thus Dacia came under Gepidic domination as early as 455, and not, as archaeologists once believed thanks to a superficial assessment of their finds, many decades later. This is clearly demonstrated not only by the account drawn from Priskos but also by the archaeological finds. The archaeologists had concluded erroneously that in Dacia, only the inner Transylvanian region that happened to yield relevant finds had been occupied by the Gepids. Yet the original written sources are unambiguous: the Gepids' military domination extended along the Lower Danube to the mouth of the Olt, and therefore it must have encompassed all of the former provinces of Dacia, including Dacia Inferior. According both to historical {1-187.} records and to Hun archaeological finds, Oltenia had been a 'Hun settlement area'. The account based on data from the contemporary Cassiodorus clearly defines the Gepidic borders in the first third of the 6th century: 'Scythia prima ab occidente gens residet Gepidarum, que magnis opinatisque ambitur fluminibus. Nam Tisia per aquilonem eius chorumque discurrit; ab africo vero magnus ipse Danubius, et ab eoo Flutausis secat, ...intorsis illis Dacia est, ad coronae speciem arduis Alpibus emunita'.[11]11. Jordanes, Getica, 33. In other words, the Gepidic people live in a region west of Scythia Minor (Dobrudja), bordered by great and famous rivers, in the north by the Tisza, in the south by the great Danube itself, and in the east by the Olt ... that flows into the Danube. Beyond these, protected by the high Alps, lies 'Dacia' — which in this formulation does not coincide with the territory enclosed by the Gepids' political frontiers. The same boundaries are noted by the contemporary Jordanes: 'Daciam ... quam nunc Gepidarum populi possidere noscuntur, quae patria in conspectu Moesiae sita trans Danubium coronae montium cinguntur ... haec Gotia, quam Daciam appelatur maiorem, quae nunc, ut diximus, Gepidia dicitur ... a meridiae Danubii terminabant'.[12]12. Jordanes, Getica, 74. Thus the Gepids' present-day country lies opposite Moesia, on the other side of the Danube. The country whose name was once Dacia, and later Gotia, is now called Gepidia and is bordered in the south by the Danube.

The Gepids must have exercised firm control over the borderline formed by the Danube between the Tisza and the Olt in order to be able to wage a successful war in 539 in the Lower Danubian region. Allied against Byzantium with the Frankish King Theudepert, the Gepids crossed the Danube and (while their allies attacked the Byzantines in Italy) won a bloody and devastating victory over the Byzantine army of magister militum Calluc, killing the latter in the process. They thereupon occupied 'almost all the Dacian towns'. This refers to the Eastern Roman 'Dacia ripensis', which stretched along the right bank of the Danube from the Iron {1-188.} Gates to the mouth of the Olt; but the Moesian Singidunum (Belgrade and its surroundings) also came under Gepidic rule. This was the occasion when, for the first time, the Gepids acquired a sizeable 'enslaved Roman population'. Over some ten years, the Gepids would facilitate the movement of Slavs across the Lower Danube, and in 550 they helped the Kutrigurs to attack Thrace. The Gepids surrendered former Roman territories south of the Danube only after being defeated by a Byzantine-Langobardic alliance in 551.

Dacia Inferior (Wallachia, Oltenia) was a military border zone, and in keeping with their settlement pattern, few if any Gepids made their homes in this region. However, the fact that only a bare trace has been found of the Gepids' passage (a 6th-century stamped potsherd, at Şimnic near Craiova) owes more to the insufficiency or bias of archaeological research. Jewels, weapons, and vessels of the Gepidic occupiers have been discovered on both sides of the Lower Danube, at Orsova (a grave containing fibulae) and other sites, including Singidunum, Viminacium, Ratiaria, and Augustae. The scarcity of archaeological finds reinforces the hypothesis that during their rule, the Gepids did not allow a significant native population to remain in the area enclosed by the Olt and the Lower Danube. The Gepidic 'Olt limes' was broken through in the early 560s by the Slavs or, at the latest, in 567–58 by the Avars.