All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger. Decius (Daqyanus
Diqyaanoos (Decius) was a king of the Romans, as stated by Shaykh Ibn Taymiyyah and it was during his reign that the people of the cave came out, as a group of scholars of Tafseer (Quranic exegesis) stated.
We have not come across any scholar who wrote a biography about him or said who he was, but the book At-Tahreer wa At-Tanweer (15/262) reads, "He was a king in the year 237 (A.D.) His reign lasted one year."
Allah knows best.