When Rachmaninov composed this set of variations over a theme by the famous virtuoso violinist Paganini in 1934, he was following a well known path. Brahms and Liszt were already before him, and many would follow. The Capriccio no 24 draws in composers ever since. Even Paganini himself used his own theme to make diabolical variations. It is perhaps the most varied-upon theme in musical history.
Paganini used these Capriccios as his calling card: he is showing off. People said at the time that for this kind of talent, he must have sold his soul to the Devil. This Beauty and the Beast story is what motivated Rachmaninov. In his variations he created a contrast between the theme of Paganini, and a theme close to his heart: a medieval tune called Dies Irae. It personifies the Devil, and is fighting a battle throughout the variations.