Let's imagine a high magic world. Most of the cities, resources, armies, arms, and everything else is controlled by guilds.
There is a small generational monarchy that owns little but has a lot of power. They have their own castle, some clerks, scholars and a few hundred honor guard to keep the peace. Any guild could overrun them with military power without any issue. What the monarchy has is access to immortality. This is a gift of their bloodline, and can be bestowed upon others. Any one receiving the gift stops aging for ten years, and can't be killed unless most of their body is destroyed. Without the gift they go back to normal, to live out their life from where their aging stopped. There are no other magical ways to significantly prolong your life, except the gift. it is impossible to separate the gift from the person, and there is a limit to how many people can receive the gift from one individual. The monarchy grands this gift for extraordinary cost only affordable by the highest members of guilds. These costs are so high that it generates enough income for the monarchy to be the richest entity.
The question is, is this feasible? Could a small group of people without a large army, religion, or a large civil presence maintain law and control if not management over very rich, organized, and greedy entities by holding eternal life over their heads?