story identification - Book (online?) where one common (and powerful) form of magic consists of making vows

I was reading Sew You Want to be a Hero, second book of the Threadbare series, and there's a scene where a character is commenting on how the Knight class (it's LitRPG where people are aware of their game stats) gain power by taking a vow and sticking with it, and it reminded me of another (non-LitRPG) work of fiction where there were several forms of magic, but the most common accepted one involves taking self-denying vows of poverty, chastity, etc to gain fighting prowess (and no, I'm not thinking of The Book of Exalted Deeds) with one particularly powerful warrior having given up eating and drinking for some decades (he wore a sort of armor that kept him from malnourishment, but does not alleviate the hunger and thirst). Initially, it's described only as being male warriors, but it later turns out that a set of religious sisters have a similar powerset owing to privation. A key factor in both forms is that you gain more power the longer you keep your vow, but the moment you break it, you're back to square one.

There were other forms of magic, one being some sort of "forbidden magic" that still works on sacrifice, but allows you to use the (possibly unwilling) sacrifice of others, and a lost art of enchanting objects (which is where the previously mentioned magic armor comes from).

I read this in the last decade, but I can't say where.

Answer

Ah, I found it. The Dark Wizard Of Donkerk by Alexander Wales (summary from here, although I wrote that too)

The basic concept of the plot is that Omarr and Hirrush, two dark wizards in the medieval kingdom of Donkerk, have acquired an infant boy from a local orphanage and plan to sacrifice him for great power. And, well, neither one of them can go through with it. And so they christen the child Henry and begin raising him in their ways, with the help of a local witch, Adrianna. Complicating things, they decide on one last kidnapping (previously their way of raising money), this time of the royal princess (due to her sex and her older brother, they assume that she will be worth a large ransom, but not enough for a manhunt). The kidnapping is successful, but not only does Henry find himself infatuated with the unexpectedly human princess, but their crime also results in a massive manhunt for black magic (it was always illegal, but largely left alone as long as they restricted their harm to voluntary sacrifice). Further complicating things, it seems that there is a prophecy that the princess will fall prey to a dark wizard, and will be saved by an orphan, an orphan born in a destined place and at a destined time that match up with Henry...

The magic I was thinking of is that of the oathkeepers (capitalization is consistently lower-case for their name):

“To raise him,” replied Omarr. “If he went back to the orphanage he would more likely than not spend ten years eating gruel and getting whatever feeble education the Sisters provide him with. When he’s ten years old, he’ll be taken to the monastery and told to swear some oaths. If he keeps them, he’ll be an oathkeeper, bound to a pitiable existence in return for awe inspiring powers. If he doesn’t keep the oaths, he’ll be sent out into the world on his own with nothing but the clothes on his back and whatever he can steal.” Omarr had run away the day before initiation, but he’d seen the path that was being laid out in front of him.

....

One of the perks of being an oathkeeper was the fact that if anyone questioned whether you had broken your oath, you could simply make a twisting jump and land with perfect poise on the top of a two story building to answer them. The seven-pointed star on his chest signaled strongly to the people he met on the street, but not so strongly as the plainly visible power of his five oaths.

The magic armor is The Strangheid Armor.

Delland had kept drinking the wine, and even though it was likely watered, he’d become less coherent even since Ventor had entered the room. The Strangheid was one of a kind, a suit of full plate that kept the body strong without need for food or drink, but while it removed the need, it didn’t remove the desire. Though the wearer of the Strangheid would be as fit and healthy as if he were getting his daily meals, he always felt on the edge of starvation and dehydration. Delland had taken an Oath never to remove the armor, never to willingly imbibe even a drop of water, or snack on even a small morsel of food. He’d gone some sixty years without wine, and now he was trying to make up for the lost time all at once.

The Foresworn Sisters are the distaff counterpart to the oathkeepers.

“The Foresworn Sisters are one of the last lines of defense if the kingdom were ever truly in need,” said Sofia. “If there were ever a disaster, or an attack, the king would call in the sisters and they would finally make use of their gifts. In the old days they used to train more, but even now they know that’s part of their duty... ”

The forbidden magic is not precisely defined, but practitioners are referred to as "dark wizards, or black witches" and it's a form of pact magic, with the distinction basically being whether it involves human sacrifice.

“Pacts are a fine line,” said Hirrush. “If I sold a passing merchant one of our chickens, the two of us haven’t made a pact, we’ve made a transaction. If I told you that I was going to give you a cookie, that wouldn’t be a pact either, only a promise that I’ve made to you that I can freely break. For a true pact, you need some trust, and either that trust comes from neither party being able to break the pact, or from the pact holding firm for a long time. A pact needs consequences, or retribution, or something solid to make it real.”

....

“Not as such,” said Clarice. “Which is why I don’t credit it much. There are a handful of hedge witches within a day’s walk of here, but none that I know of that have done anything truly black. Animal sacrifice seems to be the worst of it. It’s not impossible that one of them has gone over the edge, I’ll grant that. And no dark wizards, to my knowledge.”

Other branches of magic include "the eloists, the elementalists, the denialists, the mentalists, the spirit callers" with the spirit callers being the lost form of magic I was thinking of.

“They were supposed to be like dark wizards that didn’t need to sacrifice anything, or oathkeepers that didn’t need to keep any oaths. The spirit callers could just tap the power of the spiritual realm directly. If an oathkeeper is worth twenty men in battle, a spirit caller was worth a whole army, because they could just call up the spirit of the battlefield for them, and it would lay waste to all their enemies.”