Dragon Slayer Magic

Dragon Slayer magic (滅竜魔法 (ドラゴンスレイヤー) Metsuryū Mahō (Doragon Sureiyā)) is a branch of Lost Magic which allows its users to transform the physiology of their body into those of a dragon. As a result of this, those with Dragon Slayer abilities are able to change their body to their own element, in both offensive and defensive styles. They can also consume external sources of their natural element to replenish their own energy. They are also naturally immune to the effects of their own element that came from both their own person and to those from other sources, but must consume it through their mouths if they are to actually absorb it into their bodies. Dragon Slayers also have a heightened sense of smell too.

There are two ways two learn Dragon Slayer Magic, the "Old-Style" Dragon Slayers learn from Dragons and "New-Generation" Dragon Slayers implant Lacrima crystals into themselves. Dragon Slayer Magic has limitless functions in Edolas, and is valued as a priceless resource for everyday uses.

Dragon Slayers are apparently quite rare; often, only a single one can be found within the membership of a single guild. Making the Fairy Tail Guild very unique as it has three on its roster.

Aside from Dragon Slayer, there are many more forms of Slayer Magic, including God Slayer, which is able to devour the magic utilised by Dragon Slayers.

"Why would a dragon teach another person how to beat dragons?"

- Lucy Heartfilia asks what we're all thinking, Chapter 1

Old-Style
The "true" Dragon Slayers, who are taught by the dragons themselves.

New-Style
Dragon Slayers who have Dragon Lacrima implanted into their bodies. It is suggested that while they have vast power, they don't have the potential of true Dragon Slayers, as any Old Style has defeated a New Generation Dragon Slayer. However, it should be noted that it took both Gajeel and Natsu combined to take on Laxus, so that might have been more a show of Natsu's potential rather than the difference between New and Old generation itself. While in theory, all Dragon Slayer styles could be replicated through a Dragon Lacrima, Darkness Dragon Slayer Magic, Light Dragon Slayer Magic, and Nothingness Dragon Slayer Magic cannot be made into an artificial version no matter what due to their power, nor can they be utilized by anyone other than their only Dragon Slayers, as the dragons who teach the style had made sure that only their single pupils could learn it, in addition to being extremely reluctant to teach the style to anyone other than their Dragon Slayers."

Armor-Style
Not a style of Dragon Slayer perse, but it doesn't truly fit either of the first two known types. These types of Dragon Slayer are similar more to the artificial-type, and obtain their powers by requipping armor made out of Dragon Lacrima. They cannot obtain Dragon Force. Jason Uley is the only known example of this type.

Types of permission needed
In addition, you cannot create a Dragon Slayer style for a dragon that has appeared/mentioned in canon, unless at the end of Fairy Tail/and or the dragon's death, a Dragon Slayer has not appeared.
 * 'Free' pertains to the canon Dragon Slayer styles and certain generous users who decided to let you folk use their fanon style. You can make your own version of these styles without any permission needed.
 * 'Ask'...well, guess. This pertains to most fanon styles. You are required to ask permission from the user who created the original article to make your own variation. If you do not ask, then it will result in a friendly warning and a notice to obtain the permission of the original creator within two days. If you do not obtain permission or do not comply, then your article will be flagged for deletion.
 * Now...'Unique' pertains to styles that the creators do not want anyone besides themselves creating and using. Exactly as it says, you cannot use these styles, nor make a variation under any circumstances- don't even think about using the same element. If you decide to go against this and make a "Dark Dragon Slayer" or something, it will result in a friendly warning to change the element within two days. If it is not revamped within two days or you do not comply, it will be deleted; no exceptions. "Being in an alternate universe" and/or "being before your Dragon Slayer is introduced" doesn't cut it. Those excuses don't justify using a fanon style that people don't want using sans themselves.

Dragon Force
Dragon Force (龍力 (ドラゴンフォース) Ryū-ryoku (Doragon Fōsu) is an advanced form of Dragon Slayer Magic. Dragon Force activates when a Dragon Slayer starts to turn their own body into a humanoid dragon. Dragon Force increases the user's power tremendously. According to Zero, it is the most powerful and final state a Dragon Slayer can attain. It increases the damage from standard Dragon Slayer attacks and causes their body to cover in scales and their Magic aura to become visible, taking the shape of a dragon. For Old-Style Dragon Slayers, certain conditions must be met in order to achieve Dragon Force state; the only one known so far would be to eat substances with powerful Magical properties, preferably those related somehow to the element they can control and consume. Natsu Dragneel (through consumption of Etherion and from the Flame of Rebuke), is the only one of the three known True Dragon Slayers to activate this power. It is likely, given the description as the final state a dragon slayer can attain, that a dragon slayer may eventually gain access to it without outside sources of power.

Trivia

 * Franz Kainu is the only known Dragon Slayer, new or old, who does not use the word "Dragon" in his attacks names.
 * It seems there are many more forms of "Slayer" Magic, these include God Slayer and Cross Slayer.
 * Enko Tatsuki is the only known Dragon Slayer who also utilises a blade.