Dragon Slayer (Magic)
Dragon Slayer Magic (滅竜魔法 Metsuryū Mahō) is a branch of lost magic which allows its users to transform the physiology of their body into those of a dragon. This gives Dragon Slayers the ability to create their own element to attack and consume their own element in order to restore their stamina. Dragon Slayer Magic also appears to enhance their user's sense of smell to hypersensitive levels.
Dragon Force is the final stage of Dragon Slayer magic, though the exact conditions to invoke it is unknown. When Dragon Force is achieved, the Dragon Slayer's skin becomes scaly similiar to a dragon, and strength, speed, and magical abilities are enhanced, improving their combat capability significantly. Natsu Dragneel has achieved this at least twice; once after eating Aetherion-infused Lacrima and again after consuming Jellal's Golden Flame of Rebuke.
There are five known wizards who utilise Dragon Slayer magic, which makes it among the most frequently occuring magic type in the series contrary to its rare status in-universe.
 Traditional Dragon Slayers
Traditional Dragon Slayers are named as such because they were taught this magic by real dragons.
 Fire Dragon Slayer
The only known user of Fire Dragon Slayer magic is Natsu Dragneel, who learned this magic from his foster-father, the dragon Igneel. Fire Dragon Slayers are capable of generate flames from their entire body and breathe fire using their Fire Dragon's Roar.
Fire Dragon Slayers possess the most destructive power among Dragon Slayers according to Natsu, as he claims during his fight with Gajeel Redfox that a flames created by a fire dragon have the capability of incinerating anything.
 Iron Dragon Slayer
Gajeel Redfox is the only known user of Iron Dragon Slayer magic, who was taught this magic by the dragon Metallicana. Iron Dragon Slayers can transform any part of their body into iron and create weapons from it in combat. An Iron Dragon's Roar is a concentrated blast of metal blades, which is the only type shown so far that takes physical form.
Iron Dragon Slayers are unique among other Dragon Slayers that the metal they generate are treated as extensions of their bodies, which they can change its shape at will.
 Sky Dragon Slayer
Sky Dragon Slayer magic has only been shown to be used by one person, Wendy Marvell who learned this magic by her foster-mother, Grandine the dragon. Sky Dragon Slayer magic is a healing magic, though Wendy is capable of expelling a tornado in combat using her Sky Dragon's Roar. This magic also uses a tremendous amount of magical energy, though Wendy is capable of restoring it by eating air.
Sky Dragon Slayers are the only types of Dragon Slayers capable of using healing magic, which is a lost art.
The capability of using Dragon Slayer Magic, in the absence of a dragon, may be artificially created by infusing Dragon Slayer Lacrima into a wizard's body. When using Dragon Slayer magic, their skin becomes scaly similiar to when a Dragon Slayer enters Dragon Force.
 Lightning Dragon Slayer
Laxus Dreyar is one such wizard, who is also the only Lightning Dragon Slayer shown in the series so far. Lightning Dragon Slayers have the capability of attacking enemies with blasts of electricity both in melee and ranged combat. Laxus's Lightning Dragon's Roar is powerful enough to contend with both Natsu's and Gajeel's Fire and Iron Dragon's Roars.
 Poison Dragon Slayer
Cobra of the Oracion Seis is the only known Dragon Slayer to utilise the Poison element. He is capable of attacking enemies with potent venom as well as expelling poison from his Poison Dragon's Roar. Although their attacks are fatal, Sky Dragon Slayers are capable of curing those affect by a Poison Dragon Slayer's venom.
Incidentally, Poison Dragon Slayers, as an extension of a Dragon Slayer's natural ability to consume their own element, have inherent immunity to poison.