He served as the Zygon commander during their plot to defeat the human race and turn Earth into a new Zygor. Some of the Zygons felt this was too risky, but he believed the risks needed to be taken. He personally thought up the scheme of killing King Edward VII and, when world leaders attended his funeral, capturing them to take their body-prints. Brelarn took the form of the lunn's dog Teazel and mauled three Zygons to death for mutiny.
After Teazel escaped, he reverted back to his original form. He attempted to stop the Tenth Doctor from subduing the Skarasen. The Skarasen saw his master and angrily ate him whole. (PROSE: Sting of the Zygons)