The Abomination was a Kaled mutant which was experimented on by Professor Weston during a war against the Daleks. He hoped to alter the behaviour of the Dalek, making it a less aggressive and peaceful creature. The Daleks were disgusted by this and organised a search for the mutant with the intention of destroying it, referring to it as the Abomination.
The Abomination was eventually found in a secret base of Weston's, but with the intervention of the Eleventh Doctor and the Space Security Service, the base was set to explode, destroying all the Daleks. When the Daleks halted the countdown, the Abomination became restless, broke free of its tank and pulled a lever which allowed the countdown to finish. All the Daleks died in the following eruption.
The Doctor did not believe the experiment would be successful but he did not witness the Abomination's actions. Unknown to everyone, the Abomination survived the eruption and waited on a ledge the magma had not reached. (COMIC: The Only Good Dalek)