Shield-maiden

The only detail that qualifies one for a shield-maiden is that she must be a woman of high status among her community, such as a sister-in-law to a priest or the daughter of a chieftain for example. There are instances of a Jarl's own daughter being selected to be his shield-maiden, a truly cruel fate to be sure: one such case is that of Hæðr Alvhankæn, who has since become known as one of the fiercest warriors of all, under Hjorald Balgrun. Shield-maidens also are only appointed to childless women who are under the age of 40, to ensure that they are at the hight of their physical health. Knowledge of magic is not required but does make one more effective, so a few maidens take up study of the mystic arts, in some form.
A woman actually has no choice in the matter of whether she is to become a shield-maiden or not: instead, she is chosen by fate if she is destined to be one, which is determined by a priest or seer. To do this, the priest must perform a ritual of reading raven bones boiled in their own marrow: ravens are closely tied to fate and the gods, and so reading such bones can give a holy person an insight into someone's fate. This ritual is a special rite among the Vörðr once they reach the age of adulthood, to give potential insight into one's destiny, which they cannot control. However, there is only one fate that can designate a woman to be a shield-maiden: if, in the afterlife, she is to become a valkjaða, a female spirit who chooses those on the dead from the battlefield.

To become a shield-maiden requires, as many things in myth do, a sacrifice: specifically, the woman must sacrifice her "love chamber," so the prayer says. What this means is that the woman must have a priest or shield-maiden cut her uterus from her body, and thus give up the ability to produce a child. This makes it so that the woman doesn't have to pick between motherhood and their maidenhood, but also serves towards ensuring she is ineligible as a potential mother and ensures her afterlife as a valkyrie. If the woman dies during the ritual, she is given a warriors funeral, in honor and respect of the sacrifice she made. It is not uncommon for a Jarl with a lot of daughters may offer some up to be shield-maidens as a means of limiting succession issues.

Once appointed to the role, a shield-maiden is completely bound to the family or house of the jarl who appointed them. However, the exact nature of their service is dependent on the oath they swear: some shield-maidens only serve until a jarl's reign has ended for example. Most though serve the Jarl's house or family until their own death, with some going on from jarl to jarl well after their lord has passed on or been overthrown. 
While stories abound of shield-maidens as traveling adventurers and heroes of the people, whether these have historical basis is disputed by scholars. Regardless, modern shield-maidens function primarily as a royal guard of sorts, protecting and taking orders directly from the jarl and his/her family. A shield-maiden's primary goal above all else is to guard her charge, but her jarl can overrule this with commands of his own, such as if he needs her to act as a commander in his army.

Even after the death of whomever, they were protecting, a shield-maiden serves until death. Once appointed, her duty is not only to her lord but also to his house: this includes his descendants even after they've succeeded him.
However, the protector in question can dismiss a shield-maiden under a few, very specific grounds: for one a shield-maiden will be punished severely if they break their oath and attempt to kill who they are meant to be protecting. If such a thing is to occur, it is not only seen as a betrayal of her liege, but also of the gods and her people. The punishment of such a crime is Sjelgot , a punishment reserved for religious oath breakers.

Shield-maidens hold a great cultural significance to the Vörðr, who are the only people of Kadagard who practice it. In the past, the Anlsax, who have subsumed into the Briane, did practice it, but the Briane did not continue this practice. Most who know the details of it use this tradition as evidence of the Vörðr being categorized as barbairians.