The Snake Summons Spell is most famously used by Draco Malfoy during a chaotic Dueling Club meeting in his second year at Hogwarts. Under the supervision of Gilderoy Lockhart and Severus Snape, Harry Potter and Draco Malfoy were pitted against each other for a demonstration duel. After Harry successfully disarmed Malfoy with the `Expelliarmus` charm, an enraged Malfoy retaliated not with a disarming charm, but by casting `Serpensortia`. The spell produced a “long, black snake” that shot from the end of Malfoy's wand and landed on the floor between the two students. The snake rose, ready to strike Justin Finch-Fletchley, a Hufflepuff student in the crowd. This event inadvertently revealed Harry's latent ability to speak Parseltongue, as he instinctively commanded the snake to “leave him,” an act which the terrified onlookers mistook for encouragement. The incident cemented the school's suspicion that Harry was the Heir of Salazar Slytherin, as being a Parselmouth was Slytherin's most famous trait. The conjured snake was ultimately dealt with by Severus Snape, who cast the Snake-Vanishing Spell, `Vipera Evanesca`, causing it to vanish in a puff of black smoke.
The difficulty of learning this spell is not explicitly stated. However, as it is a form of Conjuration—a branch of Transfiguration that is generally considered advanced magic—the fact that Draco Malfoy could cast it in his second year suggests either a particular aptitude for the spell or an early interest in the Dark Arts. The primary counter-spell is `Vipera Evanesca`, which causes the conjured snake to vanish. Other general offensive or defensive spells could also presumably be used to stun, kill, or repel the conjured creature.
The incantation `Serpensortia` is a compound of two Latin words:
Combined, the incantation can be translated as “giving birth to a snake” or “the rising of a snake.”