"He's one of my own people, Victoria, and he's hunting me."

The Darkheart is a faded neutron star surrounded by dead worlds. Except one: the last enclave of the Earth Empire, and as the rest of the galaxy enjoys the fruits of the fledgling Federation, these isolated Imperials hide a horrifying secret.

The TARDIS crew arrive to find that the Federation has come to reintegrate this lost colony. But all is not well in the Federation camp: allegiances shift, the fierce Veltrochni have vengeful plans of their own, and another time traveller is manipulating the mission for his own mysterious reasons - a true master of his craft, and a face the Doctor has not forgotten.

The Doctor must uncover the terrible secret which brought the Empire to this desolate sector, and find the source of the strange power maintaining their society. But can a Time Lord, facing the ultimate temptation, control his own desires?

This novel takes place after the Missing Adventure Twilight of the Gods by Christopher Bulis.

In Slavic mythology and Russian mythology, Koschei is an evil immortal sorcerer, menacing principally young women. Koschei is also known as Koschei the Immortal or Koschei the Deathless due to his soul being sealed inside a needle inside an egg on a hidden island, enabling him to live forever as long as the needle is intact. Given the Master's obsession for further regenerations in The Keeper of Traken and The Five Doctors, the name is apt.

• The Cloister Library - The Dark Path