In Book 1 of the Thomas Covenant series, there’s a unit of 500 elite soldiers who are sworn to immortal service to a kingdom. The soldiers don’t age: they can only die in battle, or if they break their oath.
Such a unit are just as interesting as their oath of service. They’re totally trustworthy – to follow the letter of the oath.
If they’re sworn to serve and protect the king of a country, they can’t, of course, assassinate the king – but if the king’s younger brother performs the assassination, they will immediately transfer their loyalty to the murderer. If a userper takes the throne and performs the necessary rituals of kingship, who will they follow?
If their oath is to the king’s family, not to the Crown, they will keep on fighting after the king is deposed. After all but a handful are slaughtered, a few might decide that they can best serve the rightful king by spiriting him away and going in exile with him.
Imagine a young woman who doesn’t know her parentage. She’s served by two middle-aged servants who both exhibit motherly concern for her, and who have taught her considerable skill with weapons. At a certain age she’ll start to wonder why her servants have never appeared to age, and why she’s never had to pay them.