A friend of mine told me that while the Huns did conquer large swathes of land whenever they unified under one ruler, the empire would usually fragment soon after the ruler died. Did Hun succession cause a lot of in-fighting or what else is the cause?
Follow up; what role did it play in the play in the empire's demise?
Well I cannot speak for Hunnic succession (except that after Atillas death, the hunnic empire broke apart as do so many successful barbarian empires), but I can give the example of the mongols succession. Before his death Genghis Khan had called his sons together and had them choose who among them would be the best successor. It did not immediately fall to the oldest, but had to be decided at a kurultai, which was a gathering of all the strongest families, leaders, etc. It was by a vote at these events that the next Khan was chosen. This worked incredibly well for Genghis's successor Ögedei Khan (the third born son of Genghis). As time went on the mongol empire became divided by the descendants of Genghis Khan's sons, and eventually led to non-unanimous decisions for the new Khan, then attempted assassination by the house of Jochi, then finally to civil war.
I'd also like to add that this selection process was not unique to the mongols, and Temüjin became "Genghis Khan" via Kurultai.
Edit: source "Genghis Khan: His Conquests, His Empire, His Legacy"