The Blood Seed was the plant which was inside the Magister Maleficorum.
The Legend[]
The Magister Maleficorum was supposed to contain all the evil in the entire universe. Inside the box was the Blood Seed, the fruits of which were said to grant the eater immense power and knowledge.
Series 5[]
In "The Darkest Hour" George opened the Magister Maleficorum, allowing the Blood Seed to flourish. The plant fed of the oxygen in the room and began to grow and fruit. Morgan bit into one of the fruits and was apparently consumed by the plant.
Later in the episode Morgan reappeared as part of the Blood Seed. He was defeated when the Draculas and their new allies combined their powers to imprison the Blood Seed inside a cake box.