After a brief skirmish, Stephen forced his father to cede all the lands of the Kingdom of Hungary to the east of the Danube to him and adopted the title of junior king in 1262.
Both Demetrius and Michael joined Stephen's retinue, connecting their fate, fortune and social ascendancy to the power aspirations of the younger king.
When the royal army invaded Stephen's realm in December 1264, Demetrius participated in the initial battle near Déva (Deva, present-day Romania).
Béla's advancing army, commanded by Lawrence, son of Kemény forced Stephen to retreat as far as the castle at Feketehalom (Codlea, Romania) in the easternmost corner of Transylvania.
As a result, Stephen intended to send his special envoy Demetrius Rosd to his parents, Béla and Maria, in order to seek mercy, but the besiegers captured him and Lawrence tortured the prisoner.
[7] After the reconciliation between Béla and Stephen in 1266, Demetrius was delegated by the younger king to the restored national joint judicial courts composed of the partisans of the two monarchs.
[2] Demetrius and Michael also received the right of patronage over the Cistercian Klostermarienberg Abbey (Borsmonostor, today part of Mannersdorf an der Rabnitz, Austria) sometime around 1270.