Epirus was a princedom in the area of OTL Albania / western Greece.
As in OTL, in 1224 they conquered the Kingdom Thessaloniki, and in 1227, Theodore of Epirus and Thessalonica drove the Nikaian garrison out of Adrianople and annexed much of Thrace. Bulgaria and Nikaia allied against him, defeating and capturing him and dividing his lands in 1230. But in 1241, Bulgarian czar Ivan Asen II died, and Bulgaria lost its influence on Epirus. Still as in OTL, in 1253 Nikaia attacked Epirus and Thessalia. After hard fights they conquered Thessaloniki. In 1261 they also attacked Morea, hurt the allied states of Sicily and Epirus (similar as IOTL), but couldn't establish itself permanently in Mistra.
After Constantinople was conquered by Nikaia in 1276, Nikaia's energy and power wasn't sufficient for further attacks on Epirus and other Crusader states.
In the decades following Constantine Tikh I's death in 1280, Macedonia became independent, and Epirus, Byzantium and Bulgaria would compete for this area.
In 1323, Serbia invaded the crusader states of Epirus and Thessalia, but lost them a few years later again.
But now a new great threat had come to Greece: The Rum-Seljuks. In 1374, the last crusaders were thrown out of Epirus, and the land came under Seljuk rule.