Pact of the Great Old One

The Pact of the Great Old One is a warlock pact made with a mysterious being whose nature is utterly foreign to the fabric of reality.

In Practice
Warlocks who make pacts with Great Old Ones receive alien powers from their master, typically showing up as telepathic, entropic, and mind-affecting magic. Those who make these pacts often end up going insane, as exposure to these all-powerful beings typically is too much to handle for mortal minds.

Pacts of the Great Old One have a wide variety of ways in which they are formed. Often, these eldritch beings reach out to find servants, though due to the limitations of their powers, this is often quite rare. More often, fledgling warlocks stumble across some form of connection to these beings, such as a tome that details rituals to contact them, an item imbued with a fragment of their power, or person already in contact with them. From there, pacts are made, usually binding the warlock to service to their deity in exchange for arcane power.

The Circle of Worun has banned the practice of becoming a warlock under a Pact of the Great Old One.

Pact-Makers
The most well-known of the Great Old Ones are the Elder Gods, several of the six extremely powerful divine beings who are responsible for creating the world. Currently, they are imprisoned in their own planes, unable to escape. However, four of them, (Guul, Valazar, Vorokar, and Yfuleg) have been said to utilize the Breach in the Seal to make pacts with aspiring warlocks. There is said to exist a tome called the Grimoire of Unknowable Minds that details how to communicate with these beings, though its existence (as well as the existence of the Breach itself) is the subject of much debate among members of the Circle of Worun.

The other primary source of Pacts of the Great Old One are the Elder Evils, which are powerful eldritch beings said to have the power to destroy entire planes, if not the entire universe. They include Atropus, the World Born Dead, Zargon the Returner, Pandorym the God-Slayer, Kyuss, the Worm that Walks.