The Douwd have been mentioned (in passing) in four officially licensed Star Trek books but have never appeared as a character, other than in the original TNG episode, Survivors.
Even the Q recognize a common consensual reality, a certain
metaphysical bedrock or foundation that transcends even our own
infinite command over time and space, energy and matter. The
alternative is utter chaos, and we all understand chat. So do the
Organians and the Metrons and the Douwd and all of the other truly
advanced intelligences. But not 0, not anymore. He's different
Machines were born, sometimes surpassing their makers, and fragile
life-forms dared the void between worlds in vessels of every
description, leaving their tracks on a thousand systems before
shedding their physical forms entirely to become numinous beings of
pure thought. There were the Organians, Picard realized, and the
Metrons and the Thasians and the Zalkonians and the Douwd.
Buried Age :
She had heard tales of a secretive people called the Douwd, beings to
whom imaginative thought defined reality, so that they lived lives of
illusion and pretense, rarely revealing themselves for what they
were-although the one thing they would never do was use their
deceptions to harm others.
Their frequent visitor from the Q Continuum had demonstrated his
amazing powers for them time and again. Once, he had even granted the
first officer a taste of them. “Or the Traveler,” he continued, “who
can manipulate the very fabric of space and time. And don’t forget the
Douwd, who was able to wipe out an entire race with a single thought.”