Complex sentence: He made an alliance with Matthias Corvinus in July, persuading him to release Basarab's rival, Vlad Dracula, who had been imprisoned in Hungary in 1462.
Split into simpler sentences: He made an alliance with Matthias Corvinus in July. At his request, Corvinus released Basarab's rival, Vlad Dracula, who had been imprisoned in Hungary in 1462.