Battlestar Imperial is one of the Battlestars found in orbit of the Twelve Colonies, along with the Battlestar Colonist, Gunstars Bombardier and Survivor and the aircraft carrier Hawk. It is an advanced Valkyrie-class Battlestar which served with distinction during the Thyferra cult insurrection on Tauron and during the Colonial civil war, where she single-handedly took on four revolutionist Battlestars along with their escorts and managed to destroy two before being relieved by a pair of Titan-class loyalist basestars.
Currently, not much is known about either event but what is known for sure is that Battlestar Imperial served with Battlestar Taurus during the Thyferra insurrection.
Battlestar Imperial has a pair of heavy-calibre frontal guns (Like the Mercury class, but smaller) which allow it to shred down capital ships quickly. She carries 160 Vipers during normal duties and 60 raptors, a high Viper-Raptor ratio shared by many of the Valkyrie-class Battlestars.
The Colonial Civil War is explained largely in Battlestar Phoenix: Imperial Flashbacks. Many of the Cylon War veterans became unimpressed at the new arrivals and were largely shunned from Colonial society. In return, they took their fifty one Battlestars away (To an unknown area) and returned two years later with devestating effect. They declared war on the Colonial Fleet and the war began.
Picture copyright CanisD, Battlestar Columbia picture copyright SyFy.