Book about a girl who uses artifacts to purge a dark land

Sounds like Shadow and Bone (2012) by Leigh Bardugo.

• The protagonist is a teenage orphan named Alina Starkov, a citizen of the Kingdom of Ravka.
• Within Ravka lies an area known as the Unsea or the Fold, a perpertually dark, barren strip of land inhabited by winged monsters named volcra. It must be crossed to travel between the capital of Ravka and its port cities on the western coast.

• Alina discovers that she can summon light, and becomes known as the Sun Summoner.
• The main villain of the tale is a man called the Darkling.
• The Darkling kills a magic stag, and uses its antlers to enslave Alina, though she eventually breaks free.

The Wikipedia plot summary reads as follows:

Alina Starkov is a teenaged orphan, a citizen of the Kingdom of Ravka. She grew up with Malyen Oretsev at an orphanage in Keramzin, which is sponsored by a wealthy patron. As the story begins they are marching to the Unsea (also referred to as the Fold), a perpetually dark, barren, strip of land cutting most of Ravka off from the sea. Periodic expeditions are sent across to take goods to the sea and bring back imports. The crossing is dangerous because the Unsea is inhabited by monsters named volcra. Early in their crossing, the volcra attack, and, while saving Mal, Alina displays an extraordinary Grisha talent. The Grisha are people with the ability to manipulate the elements to use as weapons, e.g. to call fire, to summon wind, to stop hearts. Alina can summon light.

The leader of the Grisha, the Darkling, immediately rushes Alina to the capital of Os Alta, saying her power is unique and valuable and makes her an assassination target by enemies of Ravka. While there she meets other Grisha. She struggles to fit in and to believe in her abilities, and begins rigorous training. She feels a strong attraction to the Darkling, which he seems to reciprocate. During two encounters they kiss, and Alina is confused by her reactions to the kisses.

After demonstrating her power to the King and his court, Alina is told by her tutor Baghra that she must flee, that the Darkling is using her, that the Darkling created the Unsea, and that he intends to enslave her and use her Grisha power to conquer the world. Baghra also reveals that she is the Darkling’s mother. Two weeks into her flight, she is nearly captured, being saved by Mal. Mal has a nearly supernatural ability to track and was sent to find her, which he did. But instead of turning her in, he helps her escape.

They decide to hunt the magic stag in the far north. If Alina kills the stag and makes a necklace of his antlers, her powers will be greatly amplified. After much time and effort they find the stag, just as they realize how much they love each other. Alina refuses to kill the stag, and the stag acknowledges this. At that moment the Darkling and his minions appear. The Darkling kills the stag and places the antler necklace on Alina, making her his absolute slave, unable to disobey him in the slightest.

They quickly return south to the major crossing point of the Unsea. The Darkling forces Alina to protect the ship during the crossing. Near the other side, the Darkling extends the Unsea, causing great death and destruction. He then throws Mal off the ship, onto the Unsea, to be devoured by monsters. In desperation, Alina finally realizes that her act of mercy, sparing the stag, gives her the possibility to break free of the Darkling's enslavement. Her love of Mal grants her the strength she needs. Alina breaks free, leaps out of the ship, saves Mal, and destroys the ship.

The book ends with Mal and Alina taking passage across the True Sea, escaping from Ravka and the Darkling.