Great Old One patron

A Great Old One patron was one possible sponsor for a warlock. If a bargain was made between a warlock and a being from the Far Realm, an elder evil, or some other ancient and mysterious being, the warlock's patron gave them access to certain powers, including a suite of spells that expanded the choices to fill their spell slots.

Great Old One Patrons
List of Great Old One patrons

Great Old One Patron Expanded Spells
Novice warlocks with a Great Old One patron could cast dissonant whispers, Tasha's hideous laughter, detect thoughts, and phantasmal force. Such warlocks with more experience could cast clairvoyance, sending, dominate beast, and Evard's black tentacles. Masterful warlocks could cast dominate person, and even telekinesis.