The only planet with a Starfleet death penalty for visiting.
Talos IV is the home planet of the Talosians, whose ancestors' war left its surface a ruin. The survivors live underground and use their powerful telepathic illusions to experience the lives of others, which made them dependent on illusions and unable to repair their own machines.
In the original pilot, The Cage, Captain Pike is lured to the planet and held there as part of a Talosian plan to breed a new race of servants with Vina, the only survivor of an earlier crash. After Pike escapes, Starfleet issues General Order 7, which forbids any visit to Talos IV and makes it the only offence that still carries the death penalty.
In The Menagerie, Spock risks that penalty to bring the crippled Pike back so he can live in the Talosians' illusions with Vina, and Starfleet chooses not to punish him. In Discovery's second season, Spock and Michael Burnham travel to Talos IV, where the Talosians help repair Spock's mind, which had been disturbed by his visions of the Red Angel.