The Unity era was a period of 189 years wherein the galaxy was dominated by the Empire of the Sith Order of Decreto. Beginning in 121 ABY following the death of Darth Abeonis, Dark Lord of the Decreton Empire and head of the Sith Order of Decreto, the Unity era was subsequently named as such following Darth Krayt's victory over his rival Darth Proditor in their war for the throne.
Throughout this "era of peace", the main focus of the Sith Empire was the destruction of the various pirate organizations and planetary governments that had formed in the wake of the Sith Crusade and the Successors' War. The era eventually ended in 300 ABY, after a sixty year war between the Sith Order of Decreto and the Zao tenj.
Jedi historians such as Tai painj would later make note that, while the Legacy era symbolized an era dominated by the Jedi; the Unity era was dominated by the Sith entirely. And so it was decided, that by simply turning the Legacy era's image upside down (with slight alterations) the Unity era could be properly represented.
- See List of Eras.