This is a basic timeline. More to be added. I won't go back 5000 years like most fantasy settings, as it would not really matter to the adventurers.
Timeline for the Empire of Honzu.
(IA = Imperial Age)
-500 IA The City of Nekoji founded.
-450 IA The neko begin trading with the human barbarians on the island.
-350 IA The City of Kujo founded.
-330 IA The City of Ichijo founded.
-300 IA The City of Honzu founded.
-290 IA The City of Konoe founded.
-260 IA The City of Gojo founded.
-240 IA The City of Takashi founded.
-200 IA An oni army nearly destroys Gojo, but is finally repelled with the aid of Honzu.
-198 IA Honzu conquers Gojo.
-93 IA Honzu conquers Ichijo.
-55 IA Honzu conquers Kujo.
0 IA Honzu conquers Takashi. Honzu Shuzo becomes the First Emperor. The City of Honzu is renamed Miyako.
9 IA Imperial armies are repelled from Sarumori.
30 IA Konoe joins the Empire.
79 IA A massive oni army rampages through the center of the Empire. It is finally defeated west of Takashi.
81 IA The Empire forms an alliance with Nezoka.
122 IA An oni army attacks Gojo, but is repelled.
157 IA A dragon flies over the city of Kujo for a few days for some unknown reason. A festival is started in its honor the following year.
163 IA A warfleet from Konoe destroys a pirate haven on the islands south of Nagazawa.
165 IA Yamashina Tetsu slays Daigo Kazuo in a duel at the Imperial Palace, to settle a border issue between House Takashi and House Gojo. Takashi wins some territory from Gojo as a result.
168 IA Pirates reestablish a stronghold on the islands south of Nagazawa.
170 IA Current year. Death of Emperor Ryoto. The Civil War begins.