Dragon Force

Dragon Force is 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 (through consumption of Etherion and from the Flame of Rebuke), is one of the three 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

 * Before "Dragon Force" was named, fans called it "Dragon Slayer mode."
 * Currently all Second Generation Dragon Slayers have Dragon Force activated when ever they perform Dragon Slayer magic. Although, none of the second gen dragon slayers actually call it dragon force. Whats more likely is that the second gen all have a similar ability that is far inferior to Dragon Force but there is no proof that it is actually dragon force other than similar qualities