Origin

Var and Dis were first, along with the primal elements. Created equal and able to create and organize new gods.

After Dis begins to rebel, or become independent, depending on your point of view, they create Time, Death, and Decay. They also begin to recruit allies, especially from among the lesser neutral gods, and the greater gods refuse to intervene on the side of Var.

Peak

In the intervening eons as the opposing powers various stages of control or liberation from the other, the entire Material 3rd plane experience cataclysms, reforming the entire face of Mundus. The very bedrock of reality is challenged and reshaped. Gods, are born, rise to power, dethroned, and killed as if they were no better than the mortals. Even entire pantheons rise and fall.

Meanwhile, the mortals and demi-mortals seek to eke out what comfort can be found in thier fragile spark of existence. Honor and glory may be the only chance they have to leave a lasting mark.

Great empires and opulent cities rise and fall, to be buried under the next. Power and knowledge of lore, craft, and arcanum are developed, wielded, and just as often forgotten and lost.

Conclusion

The gods and all other super-mortal powers (the ones that survive anyway) collaborate to defeat and bind Dis, the leader of Chaos, Death, and Decay.

In one of the epic clashes between two of the Titan Champions who are now dead and nameless, one strikes the other, the blow is deflected and instead cleaves the land in twain, leaving the two great continents, GI in the West and Agrios (Wild) in the East, as the Sea fills the nigh uncrossable chasm of the gap wound.

In order to complete the exile of Dis (and also bind her cohort of deities and powers) they (Gods and all other supernaturals) must also separate themselves from direct power in the Material Plane (also known as the 3rd plane). The Gods elevate and distance Astra, the Infernals descend their 4th plane. Feywild, Elementals, and other primal powers likewise withdraw and the 3rd plane is veiled and surrounded by Mist which (mostly) prevents Immortal and Mortal alike from traversing to and from the Material / 3rd plane. Of course there are some exceptions and work arounds, but overusing those would weaken the separation and thus the bonds that hold Chaos at bay.

The powers of the other planes are thus limited to acting through avatars, mortals who have been selected and elevated to channel the powers of the ancient gods.