In The Beginning
The planes are formed.
The baernaloths stagger forth from the mists of creation.
The baernaloths spawn the yugoloths.
The River Styx forms.
A baernaloth named Apomps creates the gehreleths from dead yugoloths and is exiled with the gehreleths into Carcerci.
The first ultroloth, the General of Gehenna, creates the Heart of Darkness to expunge the taint of law and chaos from the yugoloths which become the larvae of the lower planes.
A creature called the Maeldur Et Kavurik is told the true names of all the larvae.
The larvae migrated to the Abyss and Baator to evolve into the tanar’ri and baatezu.
The baernaloth dissappear, and the ultroloths become the leaders of the yugoloths.
The baatezu begin to explore the planes, eventually discovering the tanar’ri in the Abyss; as the tanar’ri stumble into Baator and find the baatezu.
The Blood War begins.
The archanaloths hire themselves out to either side of the Blood War, and betraying both.
The Lords of the Nine and the Princes of the Abyss (the Abyssal Lords) rise in power.
The baatezu and tanar’ri discover the upper planes and the celestials.
The celestials swoop down on the lower planes and the Blood War in a year long campaign. A truce is called by the fiends as they rout the celestials back to the upper planes, leaving only 3000 alive.
A schism forms in the celestial on how to proceed, forming the archons and the aasimon.
Concurrently yet separately, the baatezu, tanar’ri, and yugoloths begin recording their histories.
The fiends learn about the powers.
Gods of chaos and law begin to meddle in the Blood War until a power of chaos is slain and the surviving gods decided to take more subtle actions.
Mask of the Pit happens, as tanar’ri forces attempt to disguise themselves as baatezu and infiltrate the baatezu camp. They fail miserably.
The Four-Cross. Bel turns stag on the baatezu, joining forces with tanar’ri, then rejoining the baatezu, only to turn stag on his kind again for a year and a day, when he leads the tanar’ri into a long planned baatezu ambush, crippling the chaotic fiends for almost a decade. Bel is awarded Avernus, the first layer of Baator.
After millennia, spirits begin appearing from the prime material plane. On the lower planes, they take the form of larvae, which the baatezu learn to metamorphosize into new members of their kind.
The fiends discover Sigil and the Lady of Pain.
The first battle over the Field of Nettles.
The fiends begin corrupting the prime.
The Dark Eight form on Baator to deal with morale lowered by balor tanar’ri infiltrators. They take over the planning process of the Blood War. Of the original eight, only Baalzephon and Fucas still live.
The Maw, a massive crevace down the center of the Gray Waste, opens and swallows a yugoloth fortress. It is now called the Ghoresh Chasm. The second baatezu/tanar’ri truce forms and soon disintegrates.