What were Attila the Hun's accomplishments?
There is substantial argument among historians that Attila the Hun was largely responsible, directly or indirectly, for the ultimate collapse of the Roman Empire in the West. Prior to Attila's reign, the Visigoths had crossed the Danube into Roman territory ahead of the advancing Huns. During Attila's reign, he was frequently at warfare with the Roman Empire which weakened its Western front. Later, while the Romans were busy holding off Attila (which they did successfully for a time) they were so concentrated in containing Attila that they could not contain the Vandals who captured Carthage and much of Roman Africa. This area was the main source of grain for Rome; its loss was a severe blow to the Empire. Ultimately for many years, Attila's forces served as a buffer between the weakened Roman Empire and the Germanic forces which occupied much of Gaul. Attila had maintained an uneasy peace with Rome in exchange for payment of a handsome tribute which the Romans characterized as "gifts." With the sudden death of Attila, the Hunnish Empire disintegrated quickly. They no longer served as a buffer against the Germanic tribes who quickly overran the Empire and ultimately caused its collapse. So Attila the Hun accomplished quite a bit, even though much of it was indirect and unintentional.
An excellent resource you might consult is The End of Empire: Attila the Hun and the Fall of Rome by Christopher Kelly.
In a sense, Attila the Hun never really accomplished that much. He did conquer an empire, but it was very temporary. He did not set up a new civilization or leave a long lasting empire. Instead, Attila's accomplishments consist mainly of having won a lot of battles and having weakened the Roman Empire considerably.
Attila led armies that were able to rampage through much of Europe. He defeated Roman armies and took a great deal of plunder in the Balkans. On the other side of Europe, he pushed his armies all the way into France before being thrown back. He won many victories in Northern Italy. In doing all of these things, he weakened the prestige of the Roman Empire and helped to cause its fall.
Attila's accomplishments, then, were mainly military. He won battles and took plunder, but did not set up any lasting empire of his own.