Complex sentence: The eldest one, Michael Asen was proclaimed successor to the throne and co-emperor since the ascension to the throne of Ivan Alexander but his early death in battle against the Ottomans in 1355 put forward the question of succession once more.
Split into simpler sentences: The eldest one, Michael Asen, was proclaimed successor to the throne and co-emperor shortly after Ivan Alexander's accession to the throne. However, Michael Asen's early death in battle against the Ottomans in 1355 put forward the question of succession once more.