Kroagnon was a deranged architect who sought perfection for his masterpieces, resorting to murdering those who moved in.
Kroagnon designed Golden Dream Park and the Bridge of Perpetual Motion. When he created his masterpiece, Miracle City, he refused to move out, unwilling to let the residents ruin his work by moving in. He was eventually forced out, but left behind booby traps that killed many of the residents. He fled to Earth and built Paradise Towers, but when he again tried to prevent people from living in his creation, his disembodied mind was exiled to the Towers' basement.
However, Kroagnon needed a body he could put his mind into, so he began using the cleaners to kill people and deliver him the bodies. This was noticed by the Chief who made plea for him to stop, but Kroagnon simply replied that he was hungry. Kroagnon experimented with corporelectroscopy and had cleaner 479 bring the Chief to him. He used the process to put his mind into the Chief so his body became Kroagnon's. He ordered his cleaners to gas each floor of Paradise Towers in turn so that all life within was wiped out. He took up office in the caretaker base.
The Seventh Doctor sent him a message, taunting him and criticising his work. Following this, Pex came to him and promised to take him to the Doctor. Kroagnon saw it as an opportunity for revenge and agreed. When they arrived, Kroagnon found it a trap, albeit one that was not complete. The Doctor attempted to push him down a lift shaft but wasn't strong enough. Kroagnon began throttling the Doctor and smashing his body against a wall. Pex took a stick of explosive and threw himself at Kroagnon. Both fell down the shaft and the resultant explosion presumably killed them both. (TV: Paradise Towers)