Complex sentence: He is also tasked with preventing the last of the Valerious family from falling into purgatory, the family swore to kill Dracula nine generations ago and is unable to enter Heaven until they succeed.
Split into simpler sentences: He is also tasked with preventing the last of the Valerious family from falling into purgatory. The family swore to kill Dracula nine generations ago and is unable to enter Heaven until they succeed.