The ancient computer that provides for every need of the people of Aldea and powers the shield hiding their planet, built by ancestors whose science they no longer understand.
The Custodian appears in the episode When the Bough Breaks. Aldea is a legendary world, hidden for thousands of years behind a cloaking shield. Its people devote themselves to art and leave everything practical to the machine, asking it questions and receiving whatever they want.
The Aldeans can no longer have children. They kidnap several from the Enterprise-D, among them Wesley Crusher, and offer knowledge in payment, expecting the Custodian to fend off any objection; it throws the starship three days' travel away as a demonstration.
Wesley and Doctor Crusher find the cause. The shield has thinned the planet's ozone layer, and radiation is making the Aldeans sterile and ill. Picard has the power source shut down and the children are returned.