Void - What mortals perceive as the nothingness eroding the world. It is more accurately the primordial soup of realities, a seething chaos in which everything and anything constantly appears and disappears in an endless maelstrom. It is the ultimate source of all energy but it is purposeless, formless and without meaning until it is shaped by will.
Will - Coherency, the prime mover of existence. It is will that shapes the nature of the void, that forces it into patterns and structure and gives rise to reality. When the Trinity arose through random chance in the entropy of the void, it was their will that allowed them to stabilize themselves and eventually shape reality about them. With their destruction reality has lost that which maintains it and so is crumbling back into chaos. Sorcerers have the ability (though why will be left undefined) to shape the Void through application of their will, though at game start they will not understand this and will merely perceive it as the shaping of reality. As their ability to focus their will increases so will their power over the void and the reality that flows through it.
Power - Is the ability of a individual to impose their will on the reality around them. A Sorcerer's power represents a direct connection between will and reality. When they think it, it becomes. Non-magical people also have power though their will is forced on reality through the medium of armies, gold, charisma or political clout.
The Gods, creators of existence, the first wills that forged reality and everything in it. The three Gods, and hence the godsherds, will probably have aspects (eg creation, destruction & balance) which represent traits of the void from which they arose. The Gods differ from Sorcerers in that their will is reality, whereas a Sorcerer's can merely manipulate reality. Essentially the world was the will of the Gods incarnate in which all that they created lived and existed, with their destruction reality is fading as it has lost its animating force: wills powerful enough to maintain it. Although the Trinity were extremely powerful they were not so great as to be able to resist the combined might of the Sorcerers of the world and the attack upon them was enough to weaken them and allow the last of Sorcerers to break their wills and cast them down.
The Godsherds are that which remains of the Gods essentially power stripped of any driving will and aspected to the God it came from. Gaining possession of one and unlocking its power will impart a deep understanding of the nature of reality and grant the ability to wield that aspect of it with the will. Essentially becoming a God.
The world can be saved only under a single set of circumstances: three people becoming Gods and then cooperating to save the world or create a new one. PCs will then have to decide if they really want these people in charge.
I think its best not to tell people absolutely that the world can be saved. There can rumours and legends about how to save it but until they achieve the power of the Gods this should not be obvious.
I'd like to leave the motivations of the Gods ambiguous; they were neither particularly good nor very evil, they just were though its fair to say they were very alien. They were all powerful however and did manipulate mankind: there were mystical prophecies, destinies and fate. The extent to which men were slaves to the wills of the Gods should be very unclear and players will have only the teachings of the church and the words of people around at the time.
The Sorcerers (also neither good or evil but with a tendency to make things go boom) were a threat to both the Gods and reality. So for most of the history of the world young sorcerers were found by the Church and raised as priests and were taught to serve the Gods as living saints (manifestations of the Gods power). Essentially Sorcerers were brainwashed slaves of Church doctrine and their power made the Church utterly unopposable (imagine how successful martin Luther would have been if the Pope had an Orbital cannon). Of course some Sorcerers rebelled and ran from the church but were normally hunted down and neutralised. One particularly powerful Sorcerer however escaped the Church and developed a hatred for it and Gods that commanded it and enslaved mankind in the chains of fate. This was the Sorcerer who started the rebellion and eventually cast down the Gods to free the world from their (supposed) tyranny. Again it should be up to players to decide about the rightness of wrongness of these actions though this Sorcerer will have survived (in clicherific style) and will actively oppose any who try to resurrect/become the Gods.
The universe the Trinity created is rather shoddy and small because the Three simply didn't care about making anything other than a playground for humans. It is “geocentric” and the celestial bodies lie about 10 miles above the surface of the planet moving in rings upon a a great crystal sphere that divides creation from the Void.
Stars are essentially small burning spheres about the size of a human head.
The Sun is slightly bigger, about the size of a castle, and considerably closer to sea level.
Sorcery is the process by which the void is manipulated by an act of will. It is capable of virtually anything the Sorcerer can conceive of (which is a very important factor) but it is limited by the will of the person wielding it. A person who has just unlocked their Sorcerous power can preform relatively minor miracles. An Archsorceror is a master of reality itself and there is very little difference between their thoughts and existence since one controls the other.
The Sorcery system is by design vague because I want players to feel that they can attempt absolutely anything they can think of. It also fits with the setting where there are no helpful mystical masters, Magical Schools full of angsty teenagers, or spellbooks. Sorcerer's are independent entities and magic is a deeply personal thing entirely defined by the mind of the individual who wields it, they do not learn about it they just learn it. The levels of power are intended to represent both the aspects of reality which a Sorcerer can manipulate and the scale on which they can do so. The idea is supposed to based around what the Sorcerer is able to conceptualise and force into being with their will, the increase in power is supposed to proceed approximately exponentially.