Short answers, pulled from the story.
Modern scholars use the phrase Empire of Thessalonica to describe a specific Greek state that existed between 1224 and 1246. Historians like John Arthur Garraty and Alexander Vasiliev apply this label to distinguish the Komnenodoukas dynasty's rule from other Byzantine successor states.
Theodore Komnenos Doukas captured Thessalonica in 1224 after expanding his territory into Thessaly. This strategic victory gave him access to the second most important city of the former empire following the fall of Constantinople in April 1204.
The Battle of Klokotnitsa took place in 1230 and resulted in total destruction of Theodore's army. Theodore himself was captured by Bulgarian forces and later blinded, ending his physical ability to rule.
His brother Manuel Komnenos Doukas succeeded him but could not prevent the loss of most conquests in Macedonia and Thrace. Manuel was deposed in a coup by Theodore in 1237 after Theodore secretly returned to Thessalonica.
Nicaean supporters inside Thessalonica opened a gate and let the army enter in 1246 when Demetrios Angelos Doukas refused to pay homage to Emperor Vatatzes. Andronikos Palaiologos became governor while Demetrios received estates in Asia Minor for comfortable exile.