Strigoi are vampires that distinguish from Moroi in many ways. They are undead, and they are made and not born.
While Moroi are very pale, the Strigoi's skin is chalky white and they have a red ring around their pupils. They no longer have a "mortal" personality and only take the role of an "immortal" personality by being vain, cruel, and sadistic with no feelings. Some can have skin that burns very easily in the light.
Humans, dhampirs and Moroi can be turned forcibly. There are two ways to make a Strigoi: If a Strigoi drinks blood from his victim and then forces that person to drink Strigoi blood then he or she becomes a Strigoi. The other way to become Strigoi is by choice, if Moroi purposefully kills another person while feeding. A Moroi that turns Strigoi loses his or her magical ability and can no longer go into the sunlight or walk upon holy grounds.
Strigoi are very strong physically and incredibly fast. A Strigoi's strength and immortality stemming from their diet of Moroi blood; It is Moroi blood that makes them so powerful. Newly changed Strigoi can still pose as threat with their great strength. Strigoi are strong and fast enough to swiftly snap the necks others if their victims don't take notice of them. Most of the younger novices are too inexperienced and often make mistakes, involving them underestimating Strigoi, Mason's death was an example. Trying to save Rose's life, he rushes in and the Strigoi easily stops him, snapping his neck right after. Mostly exceedingly well trained novices and guardians are the ones that can hope to match or even defeat a Strigoi. The senses they have are very acute, much more accurate than those of a Dhampir or a Moroi, making it very difficult for even the most experienced of guardians to sneak up on them. The bite of a Strigoi gives the victim an even greater high then that of a Moroi because of the endorphins in their bite being different and more potent. A Strigoi's movements are noticeably graceful.
When a Strigoi is first made, they are awkward with their new levels of strength and speed but as time goes by, they become accustomed to their enhanced abilities and become even more deadly.
A Strigoi's thirst for blood is stronger then a Moroi's is for human blood, but can be satiated with human, Dhampir or Moroi blood. Moroi blood however is what they truly crave and desire the most since it is Moroi blood that gives them so much power.
While there are three ways to kill a Strigoi, a silver stake through the heart, decapitation, or setting them on fire, there is only one way to save them. A silver stake must be charmed with spirit and then a spirit user, being the only ones who "can create life" with their magic, must then stake the Strigoi through the heart, this is seen in Spirit Bound where Lissa stakes Dimitri Belikov in the heart thus restoring him to his Dhampir state and in Last Sacrifice when Robert Doru stakes Sonya Karp, a former Moroi turned Strigoi.