Dragon Force

Description
Dragon Force activates when a Dragon Slayer starts to turn his/her own body into a humanoid dragon, gaining reptile-like scales and traits, such as further elongated and sharp canines for Old-Style Dragon Slayers, and scaly arms, sometimes complete with claws, for New-Style Dragon Slayers. According to Zero, Dragon Force is the most powerful and final state a Dragon Slayer can attain, this granting power comparable to that of a real dragon, and the ability to destroy everything.

Abilities
Dragon Force greatly increases the damage done by standard Dragon Slayer spells, and grants the user access to more advanced, powerful attacks. It also drastically enhances the user's physical prowess, making them far stronger, more resistant and faster, and boosts their Magic power, prompting it to violently become visible, taking on the shape of a dragon; when used at the fullest, the generated silhouette seems to gain corporeality, likely as a result of the immense amount of Magic power employed. When Natsu Dragneel achieved such state through the use of Jellal Fernandes' Flame of Rebuke spell, his flames also gained the ability to devour offending Magic, even that of Zero's ultimate attack Genesis Zero.

Achieving

 * For Old-Style Dragon Slayers, those who have been taught their Magic by a real dragon, 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 somehow related to the element they can control and consume; while other, unrelated substances can be eaten as well, these have been described as being similar to "poison" for the user, causing heavy after effects, such as pain on activation and extreme exhaustion after use. However, it is likely that, due to it being considered the final state a Dragon Slayer can achieve, Dragon Force may eventually be attained without outside sources of power. So far, Natsu Dragneel is the only Old-Style Dragon Slayer to have achieved such condition, doing so on two occasions: the first through the consumption of Etherion, and the second from Jellal Fernandes' Flame of Rebuke.
 * For New-Style Dragon Slayers, those who have obtained their powers by implanting Dragon Lacrima in their body, Dragon Force mode is seemingly entered whenever they make use of their Dragon Slayer Magic. However, due to this easiness to achieve this form, their powers don't seem to be increased at the same level of Old-Style Dragon Slayers, with no dragon-like aura appearing for them. Laxus Dreyar and Cobra have been shown achieving such state.