Complex sentence: While this is happening Merrin Meredith, and his small army of Things, creates a Darke Domain which threatens to engulf The Castle and all its residents plus, to add to Septimus' troubles, a ferocious dragon is on the loose.
Split into simpler sentences: While this is happening Merrin Meredith, with his small army of Things, creates a Darke Domain which threatens to engulf The Castle and all its residents. Furthermore, to add to Septimus' troubles, a ferocious dragon is on the loose.