Doctor Who Armageddon Factor by Terrance Dicks

In this narrative, the Doctor and his companions, Romana and K9, conclude their search for the Key to Time amidst a devastating nuclear war between the planets Atrios and Zeos. They discover that the conflict is being manipulated by a sinister agent of the Black Guardian known as the Shadow, who has imprisoned Princess Astra. The Doctor learns that the Princess is actually the final segment of the Key, and he must outwit both the Shadow and a rogue Time Lord named Drax to assemble the artifact. After resisting the Key’s corrupting influence and thwarting a deceptive apparition of the Guardian, the Doctor chooses to disperse the segments across the universe once more. This selfless act restores Princess Astra to her human form and ensures the powerful relic cannot be used for universal chaos.