The main rival of the Empire of Nicaea was the Despotate of Epirus. In 1224, the Despotate of Epirus contested Nicaea's claim to be the continuation of the Byzantine Empire and became the Empire of Thessalonica, but was forced to renounce their claim in 1242.
Reclaiming the Throne: The Empire of Nicaea and the Resurgence of Byzantine Power (1204-1261) - Test Your Knowledge!