The homeworld of the Zalkonians, whose government hunted down citizens beginning to evolve into beings of energy, including the amnesiac found by the Enterprise-D in 2366.
Zalkon is the home planet of the Zalkonians, a humanoid people who had had no contact with the Federation before the third-season Next Generation episode Transfigurations.
In 2366 the Enterprise-D rescues a badly injured man from a crashed escape pod in the Zeta Gelis cluster. Nicknamed John Doe, he remembers nothing, heals with astonishing speed and gives off bursts of energy that can cure others. As his memory returns he realises he was fleeing his own world.
A Zalkonian warship commanded by Sunad then arrives and demands him as a condemned criminal. John explains that some Zalkonians were undergoing a transformation which the planet's leaders feared and answered with executions. He completes the change into a being of pure energy before Sunad's eyes and returns to Zalkon to tell his people what awaits them.