From ThresholdRPG Wiki

The Hegemons are not an official organization or society. Hegemons are simply the most powerful mortals in the realm, and by virtue of their power, they are recognized by all individuals for the status they have achieved.

Every mortal that achieves at least level 20 and guild level 20 has the chance to become a hegemon based on their accomplishments and deeds, and this entitles them to certain benefits. For one, they are "famous." This means they are easily recognized and also that they have access to important documents. They can also send/receive tells to/from anyone.

For the purposes of rules like help pk, anyone who has ever been level 20/20 for any period of time is considered a hegemon.