Crime and punishment
Landak was originally a Zygon, who attempted to steal the form of another member of his species. As punishment, he was exiled from Zygor, wearing a crude and stunted form which was incompatible with body-print technology.
After having obtained a diadem which allowed him to move into the minds of others, he followed Haygoth's crew to earth. He hoped he might steal a Zygon body. Landak grew a parabolic diverter in the hope of capturing a healthy alien host.
He approached Haygoth, who was now under the ageing guise of Patricia Ryder, and grew a body-print machine which could cure his photomelanoma. This offered Haygoth the chance to become a Zygon again, after having spent two decades in the form of Patricia. Instead of accepting, Haygoth stole the parabolic diverter and ran to spend one last Christmas with Lucie Miller in Blackpool. (AUDIO: Death in Blackpool)
In pursuit of Haygoth
After having taken the body of Natasha, he attempted to follow Haygoth to Blackpool. He hitched a lift with Billy Burnham. However, upon realising that he was only going as far as Warrington, Landak attacked him on the M6 approach road and entered his mind, killing Billy in the process.
Having taken Billy's body, Landak then took Billy's car and followed Haygoth to Nadir Motorway Service Station; there, Haygoth, under his guise as Patricia Ryder, met up with the Eighth Doctor, Lucie Miller and an unemployed Father Christmas. "Billy" then followed the group to Blackpool, where he ran over Lucie as she was leaving an off-licence. (AUDIO: Death in Blackpool)
Death in Blackpool
With Lucie in a coma and transferred to Blackpool General, "Billy" snuck into the hospital posing as a porter. He entered one of the store cupboards and, using a Zynog diadem, projected himself into Lucie's mind. As Billy, Landak attempted to depress her enough that she would give up fighting, allowing him to take control of her body. He showed Lucie a conversation between the Doctor and Patricia, from which Lucie learned of Patricia's death in 1984, and that the only Auntie Pat that she had ever known was in fact a Zygon.
Haygoth entered Lucie's mind and freed her from her coma. Landak then attacked Haygoth and metaphysically choked him to death. Landak awoke in the Haygoth's body, still disguised as Patricia Ryder. However, before Haygoth had entered Lucie's mind, he had poisoned his own body with a concentrated saline solution; the body which Landak now awoke in was on the brink of death. The Doctor held his hand as he died. (AUDIO: Death in Blackpool)