Dragon Slayer Magic is a type of Lost Magic which allows its users to transform their physical bodies into a Dragon. As a result, the user can transform their body with features of their respective element, utilizing both offensive and defensive styles. A Dragon Slayer is immune to the effects of their own element, and can consume external sources of their element to replenish their strength. However, a Dragon Slayer cannot consume the element that they produce themselves, and elements must be consumed through the mouth. It is known that certain Dragon Slayers have been able to consume elements other than their own, allowing them to combine both elements into one inside their bodies and use them to increase their abilities. Dragon Slayer Magic is the only way to kill, or even hurt Dragons, who are shown to be mostly unaffected by other forms of Magic.
There are only two ways one can learn Dragon Slayer Magic. The first way is one must be taught by an actual Dragon. The ones who learned their Dragon Slayer Magic through this method are referred to as First Generation Dragon Slayers. The second way is one must have Dragon Lacrima implanted into their body. The ones who learned their Dragon Slayer Magic through this method are referred to as Second Generation Dragon Slayers. There is also another way which is a combination of the above two methods: ones who both learned their Dragon Slayer Magic by receiving the teachings of a Dragon and also having Lacrima implanted into their body. The ones who learned how perform Dragon Slayer Magic by utilizing both methods are referred to as Third Generation Dragon Slayers.
Dragon Force is the final, ultimate state that a Dragon Slayer can attain. When one enters Dragon Force, their skin becomes scaly like an actual Dragon's. The exception to this conceived notion are the Third Generation Dragon Slayers, who have scale-esque designs instead of having Dragon scales appear on their bodies. The exact conditions required for a First Generation Dragon Slayer to achieve Dragon Force are currently unknown. Natsu Dragneel, however, has achieved this state at least twice: once after eating Etherion-infused Lacrima, and again after consuming Jellal's Golden Flame of Rebuke. Wendy Marvell, on the other hand, has also shown to enter Dragon Force by consuming air that is Ethernano-abundant. Third Generation Dragon Slayers, however, can enter Dragon Force of their own free will. Though with training, First Generation Dragon Slayers can achieve this feat as well.
Excessive use of Dragon Slayer Magic has the added drawback of turning the user into an actual Dragon. The only way for this to be prevented is for a Dragon to enter the body of a Dragon Slayer (via a secret art) and sleep there for an extended period of time, creating antibodies which will almost ensure that the Dragon Slayer does not turn into a Dragon.