Warning: SPOILERS ahead for Shadow and Bone season 1.
Shadow and Bone's brooding, mysterious General Aleksander Kirigan is a unique and powerful Grisha also known as the Darkling. Descended from one of the very first Grisha, Ilya Morozova, Kirigan has the ability to summon and shape darkness according to his will. And as Alina Starkov learns in Shadow and Bone's mid-season twist, Kirigan is actually the Dark Heretic - the Grisha who created the Shadow Fold hundreds of years ago.
Most Grisha limit themselves to practising the Small Sciences, which is the manipulation of existing matter (like the Inferni throwing fireballs, or Alisha using her Sun Summoner abilities to draw in and shape light). The Darkling, however, has dabbled in the forbidden magic of merzost, which creates new objects and beings from nothing and is wildly unstable and unpredictable. Though merzost is extremely powerful, it also comes at a cost to anyone who dares to wield it.
Like all Grisha, the Darkling can increase his abilities using amplifiers like Morozova's Stag, a magical creature created through the use of merzost by Kirigan's ancestor, Ilya Morozova. But even without the use of amplifiers or merzost the Darkling has some extremely powerful tricks up his sleeve. Here's a breakdown of General Kirigan's Darkling powers in Shadow and Bone.
Summoning & Manipulating Darkness
The Darkling is among the Etherealki class of Grisha, whose powers are based around summoning of the elements. There are more common types of Etherealki: Inferni have control over fire, Squallers have control over air and wind, and Tidemakers can manipulate water. General Kirigan is the only known living Shadow Summoner (though his mother, Baghra, secretly has the same abilities). The most basic use of his powers is to summon darkness, as seen when he prepares for a demonstration of Alina's Sun Summoner powers by making the surrounded environment completely dark. The obvious applications of this in combat are to provide cover of darkness for an advancing army, to blind opponents in a fight, or simply to instil fear.
The creation of the Shadow Fold was a corruption of this ability to summon darkness. The Small Sciences wouldn't have been able to form such an enormous, permanent and self-sustaining scar of darkness across the land, but the Darkling used a merzost incantation found in Morozova's notes to try and convert the king's soldiers into his own army. It worked (kind of) by transforming the soldiers into volcra, monsters that thrive in darkness, but it also spun out of the Darkling's control and exploded outwards, creating the Fold. Ironically, the Darkling is in more danger than anyone else when he ventures inside the Fold, because the volcra are drawn to the darkness within him. But by taking control of Alina's Sun Summoner powers, Kirigan hopes to bend the Fold to his will.
The Cut
A rare and deadly ability that Kirigan only uses as a last resort, the Cut is a Shadow Summoner move that forms the darkness into blades which can be flung through the air. The Cut es smoothly through flesh and bone, slicing anything or anyone unfortunate enough to get in its way. This is first seen when Kirigan uses it to save Alina from an attacking Drüskelle (witch-hunter) by gruesomely cutting the man in half. He also uses it to behead an entire unit of soldiers in the flashback to his past, and to cut the head off Morozova's Stag.
Eternal Life
As Baghra tells the Darkling when warning him away from the use of abominable magic: "The Small Sciences feed us; merzost feeds on us." As Grisha train and grow their powers, or use amplifiers to make them greater, their bodies are rejuvenated by the use of the Small Sciences. The more powerful a Grisha is, and the more they make use of their powers, the longer their lifespan can stretch. General Kirigan is on par with Alina Starkov as one of the most powerful Grisha who have ever lived, and this is what has allowed him to survive for hundreds of years without appearing to age at all.
Exactly how long the Darkling could live is uncertain, but Baghra tells Alina that he is "eternal," implying that he could potentially live forever. While Baghra herself has aged in the hundreds of years since the Shadow Fold was created, her son doesn't look a day older. That could change, however, if he continues to pursue the use of merzost. This forbidden magic drains a person's life force instead of fuelling it, so by using it too much the Darkling could end up killing himself - or at the very least, he could lose his Byronic good looks.
Creating Shadow Monsters
Using a power glimpsed at the very end of Shadow and Bone season 1, the Darkling can use merzost to create life from nothing. But instead of creating majestic creatures like Morozova's Stag, Kirigan makes shadow monsters called nichevo'ya. After surviving being attacked and carried off by a volcra in the Shadow Fold, the Darkling emerges on the other side of the Fold and calls upon a number of nichevo'ya to follow him out into the world - something that the volcra cannot do, because they are afraid of the light.
The creation of niche'voya is distinct from the Cut because they are actually a form of life (albeit not a very pretty one). And which merzost may appear to be the creation of something from nothing, there is actually always a price to be paid for using it. In the Darkling's case, creating the nichevo'ya comes at a price to his own life force each time he does it - but given how enraged he was by having his plans foiled, Kirigan may no longer care about the personal cost of using dark magic.