Duxus

It was the highest-ranking peer of the Rexin after the royal family.
  They were the rulers of the provinces and the superiors of the Civino in the cities.
  It was a hereditary title passed down according to the laws of the Act of Blood.
Real-Life Equivalent
Duke
Meaning
'dux' duke (latin) - 'unus' one (latin)
Type
Nobility, Hereditary
Status
No longer in official use since 341
Form of Address
His Grace / Her Grace
Equates to
Duke / Duchess, Daimyo '大名'
Source of Authority
Reports directly to