Very little is known about the Douwd. So far as is known, only one individual has ever been encountered. This incident occurred in 2366, when a Federation colony was attacked and destroyed by an unknown assailant. On arrival the Enterprise discovered the planet had been almost totally devastated, with a single structure surviving. Within this house were Kevin and Rishon Uxbridge, members of the colony who had apparently survived the attack. Kevin explained that although he did not know why they had survived, it may have been because he had refused to fight in the battle against the attackers. He repeatedly refused any help from the Enterprise, declaring that he just wanted to be left alone. 
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During the investigation the Enterprise encountered a large alien vessel which attempted to lure or drive the ship away from the Rana IV colony. Captain Picard eventually allowed the vessel to destroy the Uxbridge home, then destroyed the alien ship in turn. He took the Enterprise to a high orbit and waited; after several hours the Uxbridge house reappeared. Picard beamed Kevin and Rishon to the bridge and confronted Kevin, demanding to know the truth of what had happened. 
Kevin explained that he was a Douwd, an 'immortal being of disguises and false surroundings'. He had been travelling in Human form in 2312 when he had met and fallen in love with a Human woman, Rishon. He had lived as a Human since that time, never revealing his true identity to her. When the aliens, named the Husnock, attacked the colony Rishon had gone to fight them but Kevin had refused for reasons of conscience. Rishon had been killed in the fighting. The loss drove Kevin into a terrible rage. Using his special abilities he had destroyed the entire Husnock species in retaliation, killing fifty billion people in an instant. 
Kevin was consumed with guilt over his actions, and had recreated his house and Rishon in an attempt to continue something approaching a normal life. Captain Picard decided that Kevin could not be held to account for his destruction of the Husnock, and allowed him to return to the planet to live out his fantasy.