Immortal, near-omnipotent beings, one of whom hid as a human and, in grief, wiped out an entire species.
The Douwd are immortal energy beings able to take any form and reshape matter at will. Only one has been met, living as a human named Kevin Uxbridge on the Federation colony of Delta Rana IV. Sworn to pacifism, he had married a human woman, Rishon, and refused to use his powers even when the colony came under attack.
When the Husnock destroyed the colony and killed Rishon, his grief and rage led him to annihilate the entire Husnock species in an instant. He then recreated Rishon to live with his guilt. Picard concluded there was no law that could judge such a crime and left him to his self-imposed exile.