This is an OC concept from /mlp/; her name is Second Chances (or just “Chance” for short).
 
The form of magic that she casts is called vivimancy, as in the opposite of necromancy. Specifically, instead of using her horn to manipulate traditional magic, she uses it to manipulate lifeforce. This requires a fundamental rewiring of the relevant systems, though, so as a result she can’t cast normal spells, not even illumination or telekinesis; she can’t even channel enough magic to make things stick to her hooves.
 
The rewiring also has to happen at a young age, while a pony is still malleable. Chance did this as a filly while she was grieving the loss of a pet, stubbornly refusing to listen when her parents told her death was an unavoidable part of nature. After sneaking into several restricted sections of several libraries and archives, she used vivimancy to transfer some of her life energy to her pet.
 
Unfortunately, this more or less only acts as a magical defibrillator; it can bring an organism back to life, but it can’t undo what killed it, so her pet died again pretty much immediately.
 
As an adult, she consults with several hospitals across Equestria, using an amulet to teleport where she’s needed. The amulet needs a few days to recharge after use, so she’s only called to help with important cases with a high chance of success. Causes of death such as suffocation, electrocution, and magical attacks that leave the body relatively intact are where she excels, but she can also help with other cases if aided by unicorns with healing magic.
 
Life force replenishes over time, so she doesn’t have to worry about shortening her lifespan, but it does put limit on how much she can do at once; after bringing a pony back to life, she has to rest for a few hours. Overexerting herself could theoretically kill her, but that’s a moot point because she’d pass out long before she’d be able to use up a fatal portion of her lifeforce.
 
She can also use her vivimancy to manifest biomatter from her own life-force, but the large conversion factor between matter and energy means she’s limited to doing this on a scale of individual cells; creating one cell would leave her bedridden for a couple weeks and, creating seven or eight would leave her comatose for a few months.
 
Fortunately, being a mare, she can manifest a single cell inside her womb and allow it to implant and grow. She can also coax a pony’s spirit to enter the new zygote when she creates it, but she needs to do this at the location where the pony died. The pony’s mind is then magically imprinted onto the developing cell, and the reincarnated pony is fully aware throughout all eleven months of gestation.
 
Because of the toll that this takes on her, and the fact that she can only do it once every eleven months, she insists on only using it in the direst of emergencies, which pretty much means the death of one or more of the keepers of the Elements of Harmony.
 
In order to ensure that she’s ready if this ever happens, she is careful to never overexert herself in her job and is very careful when it comes to her health, being essentially a teetotaler when it comes to drinking, smoking, et cetera. She also avoids romantic relationships herself, because she doesn’t know what might happen if she were to get pregnant with a foal of her own and then try to reincarnate somepony into her already-occupied womb.
 
As a person, Chance is generally optimistic and cheerful, but she does feel somewhat inadequate from time to time; from the viewpoint of traditional magic, she’s less magical than an earth pony, even needing a special magical shampoo to prevent her cutie mark from fading. She often wonders if getting into vivimancy was just a foalish mistake that sacrificed more than it gained. She views death as a foe to be conquered, and can fall into the trap of focusing on her failings and limitations in that pursuit more than her victories.
 
In her spare time, she dedicates herself to studying more and more obscure texts, publishing research papers and conducting studies with the ultimate goal of curing death once and for all. It’s this work that she sees as her main calling and purpose in life, with her revival and reincarnation abilities being just a couple small steps in the right direction.