When the Devil steps down from his position, he disgorges the demon snake Slitherax from his body, and the successor of the title ingests the snake and become the next Devil. Upon swallowing the snake, the successor's body becomes much bulkier. This ritual is known to be performed every 35 years. However, Aldermach Maggotbone seems to be overstaying his position since the demon snake crawled back inside him after his successor proved to be unsuitable.
What position of power the Devil holds in Hell is unclear. While he is the leader, he does not own Hell, hinting that this title is comparable to a monarchy title in a modern state.