Diegis was a Dacian chief, general and brother of Decebalus.[1] He served as his representative at the peace negotiations held with Domitian in 89 AD. After the peace negotiation, Domitian placed a diadem upon Diegis' head, symbolically saying that he held the power to bestow kingship to the Dacians.[2][3] According to Gábor Vékony, Decebalus never held the royal title.[4]
...it becomes evident that, behind King Diegis (and High Priest?) stood Decebal, who did not use the royal title but held the acutal power.