Historical Timeline of Elora
To explain how years work, everything prior to the Cataclysm is BC (Before Cataclysm). After that, the timeline is split into "eras", which are chunks of about 500 years. "PC" is occasionally used to mean "Post Cataclysm".
Note for Players: This is spoiler-free!
Pre-Cataclysm / Cataclysm
There have not been very many events that have been reconstructed from this time period as of 64 E4 (current year in the Aelonaran calendar).
Year -1 BC
A group of adventurers traveled across the land now called Aelonara to apprehend a power hungry mage attempting to ascend to godhood. Much of their plot involved political assassinations and powerful artifacts.
The adventurers in question are known to the Church of Allani as Saint Rook, Saint Randall the Dragonslayer and Saint Xaer the Spellthief.
Year 0
The powerful mage made it to the top of the highest peak in what is now the Erevan Dominion and performed his ritual and monsters from all of the planes started flooding Aelonara. Before his ritual could be completed, the heroes slayed him and sealed him away in a tomb on the peak of the mountain and broke the crystal key to prevent anybody from reopening it. This mage is known as The Forgotten One.
Even though they managed to defeat him, the ritual preparations still released unimaginable horrors across the continent that would take generations to recover from. Entire settlements were wiped off of the map and governments fell as their leaders succummed to demons and aberrations. Had his ritual been completed, the carnage likely would have spread outside of Aelonara's borders.
First Era
The first era was largely spent recovering from the damage done by the Forgotten One. The adventurers spent time organizing relief efforts during their remaining years alive and settlements would develop into cities and eventually empires.