Technologically advanced people whose cloaked world, Aldea, was thought to be a legend, until sterility drove them to abduct children from the Enterprise.
Aldea, in the Epsilon Mynos system, was long dismissed as a myth because the Aldeans hid it behind a planetary cloak and shield. Their society depends on the Custodian, an ancient computer that runs their world and that the Aldeans themselves no longer understand.
In When the Bough Breaks, the Aldeans revealed themselves because they could no longer have children, and they kidnapped Wesley Crusher and several other youngsters from the Enterprise. Dr. Crusher discovered that radiation reaching the surface through their damaged ozone layer, a result of the cloak, was the cause of their sterility, and the children were returned.