User blog comment:Uraharattack/Chapter 464: 400 Years (It's out guys)/@comment-26537859-20151130154544/@comment-1776824-20151202070815

No, he has been fighting with Igneel's power the whole time. He just released it in burst, and when he releases it in burst, it is called Fire Dragon King Mode. There is a reason those spells are called "Fire Dragon King's _______"; it is because he casts them using Igneel's, the Fire Dragon King, power. He said it took him ten months to be able to draw out the power, and that when it's gone it's gone forever. He said he could also only use "it" ("it" being Fire Dragon King Mode; the burst technique, where he releases all the power) once. Natsu has not been fighting with his own power. Why do you think when he cast those Fire Dragon King spells his eyes became like a dragon's? Natsu has normal person eyes, dude.

Also, Igneel at full strength is probably the strongest actual being in the FT series at this point in time. At half strength he could match Acnologia.