Complex sentence: The two male vampires-previously thought destroyed-have also escaped from confinement by taking the forms of the soldiers guarding them, and begun to transform most of London's population into vampiric zombies.
Split into simpler sentences: The two male vampires, previously thought destroyed, have also escaped from confinement by shape-shifting into the forms of the soldiers guarding them. The pair then proceed to transform most of London's population into vampiric zombies.