custos regni

Etymology
First attested in the plural in October 1644 and in the singular in 1683; : +  (“of royal power”; genitive singular of, “royal power”, “kingdom”) = “custodian of royal power” ≈ “guardian of the realm”.

Noun

 * 1) A kind of   appointed, either alone or jointly, to  the  duties of a  during his or her  from the.