I have a class of people in my world known as "Guardians." They are extremely powerful in this world and have special abilities that are unrivaled by almost every other class of people. They use their power to serve the people and places they are assigned to; maintain the balance among the various societies; and enforce global laws (like the ban on slavery). They are generally forbidden from participating in wars and fighting one another are also considered extremely wise and spiritual as well. Guardians are elite as only certain people can even train to achieve the rank, and even then it can take decades, and much of their power must be granted to them after they have passed many tests and proven themselves.
I'm looking to give them a title that would accompany their last name when addressed formally, in the same way you'd call a physician "Dr. Smith" or a minister "Pastor Walker"
At first I settled on "Master" as it conveyed the sense of having mastered all of the studies and skills necessary to reach the class. But, I'v since decided that this may be problematic because in my world there are many other skill sets that you can be considered a 'master' of and it took away some of the gravitas of the word in relation to Guardians who are in such a class of their own. Also I think that title might be overused.
I was considering "Elder" as it speaks to some of the spiritual aspects and makes their roll as servants of the people a bit more pronounced. But then I ran into the problem that my world also has town "elders" and again, it would take away from the title.
I'm looking to see if there is a title that I have overlooked, short of making a word up completely.
Other titles widely used in my world / story are "Chief" "Brother / Sister" and most common military ranks.