The Pact is a mystical agreement forged to prevent the manifestation of powerful extra-planar entities within the dimension colloquially known as 'Earth Prime'. It is vaguely mentioned in the Freedom City 2E sourcebook under information about Adrian Eldritch and gone into greater depth in The Books of Magic (p. 94). The specifics are specifically left ephemeral and largely up to the Game Master to enforce. In a shared world setting, such as Freedom City Play By Post, the nuances of the Pact have been influenced by characters over the years of the site's operation.
The Pact is, in fact, an ancient spell wrought by the Master Magus for Earth Prime intended to bar the gods and demons of other dimensions from treating this dimension as their playground. The power and potency of this particular spell is all but unparalleled in the time since The Pact's forging. Simon Magus was able to fundamentally alter the fabric of the dimensions to thicken the barriers between Earth Prime, and other dimensions. The intent was to bar entry to any entity that was not specifically invited; requiring a human spell worker to summon extra planar entities and for those entities to be bound by the tenants of that summoning.
In practice, The Pact is a more flexible creature. There have been many heroes, and villains, of extra planar origins over the ensuing centuries in Freedom City. Some of those have snuck their way in on technicalities, or through weak spots in the spell while others have surrendered certain powers or abilities to walk among humanity. There has been no actual sundering of The Pact, but it has been tested and manipulated; as any spell ought to be.
What does this mean for players and storytellers? Largely, The Pact is used on our site as justification for why PLX entities very rarely show up save for large scale events.
The Pact is something to take into account, especially for magic-based characters or those of divine origins, but it by no means bars character concepts from play. We have had former angels and demons as player characters, among many, many others, but The Pact provides an excellent reason for your demigod to be limited in the scope and scale of the abilities. It is often a theme that too much power risks their place in Earth Prime.
Enforcing those boundaries, or skirting them, can also make for interesting character motivations. Of the setting's NPCs, the Master Magus and Gatekeeper both take on certain duties for keeping the barrier strong, while some PC's have also taken some of that work as their bailiwick such as Phantom and the Sandman, and there is certainly room for more. It's a very large multiverse out there, after all!