A telepathic species from Talos IV.
The Talosians are a humanoid species from Talos IV with large, bulging heads and frail bodies. Their ancestors' war left the planet's surface ruined, and the survivors moved underground, where their mental powers grew so strong that they can create illusions indistinguishable from reality. Over time they came to depend on those illusions and lost the ability to maintain their own machines.
In the original pilot, The Cage, the Talosians capture Captain Christopher Pike and try to make him a mate for Vina, the survivor of a crashed ship, so they can rebuild a race of servants. Pike resists, and the Talosians let him go when they realise humans would rather die than be captive. Starfleet then forbids all contact with Talos IV under General Order 7.
In The Menagerie, Spock takes the crippled Pike back to Talos IV so he can live in their illusions with Vina. In Discovery's second season, Spock and Michael Burnham visit the planet, where the Talosians help heal Spock's troubled mind.