The blessing given to Xariah by the Light, or more often referred to by her as 'the curse', is what's shaped and directed her life since it presented itself within her in late adolescence. Only a handful of people know about the curse's existence: Xariah herself, Desmond, a handful of the higher-ups within the church, and a handful of the higher-ups within the Keep.
The Curse imbues Xariah with a few specific traits, and burdens her with a few specific behaviors; all of which are designed to support the Light's primary goal of making Xariah into an evangelist for the Light's path. An unconventional evangelist, to be sure, but one intended to reach the sorts of ears that more traditional avenues of evangelism simply can't reach.
Traits and Behaviors
The Curse grants Xariah the following:
Stasis (or Natural Healing)
Though actually older, Xariah appears to be around nineteen years old. Anything that happens to her body triggers the curse to work to restore her back to this state it chose for her to remain in. This not only includes effectively halting aging, but it also means any injuries, marks, tattoos, or brandings she might get will heal until she's back in her pristine form.
This healing is slow, but inexorable. It can and will eventually heal any injury, provided she survives it; including severe injuries like amputated limbs. This healing does not make her invincible, however; as in the short term she's about as durable as any human woman would be. A rule of thumb when it comes to injury is that if a normal human could survive with an injury for over an hour, Xariah will too, and survive long enough to recover fully; but if an injury is severe enough that it would kill someone within an hour, then it'd kill Xariah too (and she won't keep healing if she's dead).
Xariah's physical presence acts like a subtle form of influence, encouraging others to align themselves with her interests. This effect is pronounced when her partner's willpower is weakened (such as in the moments after orgasm), as it allows the influence to more easily infiltrate their decision-making.
While the Light's intention for this influence is that Xariah's devotion to the Light would make those that she's influencing also want to devote themselves to the Light; if Xariah doesn't show a particular interest in serving the Light (as she doesn't since she left the church), then naturally those aligning to her interests won't either.
The influence's effect is subtle, and no one under it would realize that she's even having an effect on them. It merely pushes them in a direction, but it leaves them believing that all of their choices are their own. How strongly the influence redirects someone depends on several factors; it takes longer to make a difference on people with stronger wills, and those that are aware of her curse and its influence can be completely immune to it.
Xariah has urges that tend to push her in directions which allow the influence of the curse to be more exposed to the sorts of people who need it. This often manifests as desire for the 'wrong' sorts of men, and desire to get into problematic situations.
This desire is like the need to breathe. Xariah can 'hold her breath', so to speak, and suppress the urges for a period of time; days, even stretching into weeks. But like holding one's breath, the longer it's done the harder the gasping for air afterward becomes. If Xariah tries to avoid entertaining her urges for too long, it'll end with her going to greater (and more depraved) lengths to feed them when her resolve finally breaks. This is something Xariah's learned the hard way, and she knows she can't hold back the river for very long without paying a price for it; and this is a large reason why she's established herself as a whore, to provide a constant outlet for the urges.
To some extent, these urges also tie into her influence. The influence tends to make people want to align with her interests, and as her need becomes greater her interests become more focused on them. This feedback loop, as frustrating as it is to her, serves to protect her as it's seen her released from captivity in the past, because eventually the growing urges that can't be sated when she's being held as a prisoner by someone will eventually encourage someone to release her (for their own prurient reasons, of course, but the end result is the same).