Igneel's and the other dragons reason for sleeping inside the dragon slayers is to prevent them from becoming dragons and go through any transformation so this theory is out .
I was confused here for quite a while and I still don't think I fully understand it yet. Honestly, I don't see why it wouldn't be Igneel's power... It doesn't look like Natsu's own power. Plus, the name of the mode and attack are the same.
I don't think there's a certain answer right now. It's a question that has popped up a number of times. Personally I think it's clear that was igneel's power. There was just too many hints there. First he used the bandaged arm, Igneel was shown before the attack and Natsu literally says he'll show them what Igneel's flames are made of. I don't think it was a coincidence that Natsu's initial attack on the at god was ineffective while the next one crushed it. It's very likely igneel's power just that he didn't release the bandages and didn't enter FDK mode so it's the same power, or should be at least
We should wait until it's said in the Manga Natsu can't use fdk spells or that it came from the power igneel gave him. Natsu saying "Let me show you the power of the flame dragon king" can just be in context igneel's magic is his magic after all. It doesn't have to mean he is literally using igneel's power. We could honestly just be trying to solve a mystery that isn't there.
I did mention that it's not certain. Well, isn't it redundant for him to have said he'd show them Igneel's power considering he'd been fighting the whole time? Heck he used an attack right before that so why would he explicitly state that he'd show them Igneel's power and why would it have a different name? Furthermore, we eventually find out that Igneel literally left power for Natsu, the latter trained with it over the timeskip and it's called FDK. Natsu had never named attacks like that prior to that.
Like I said we should wait for the Manga to say something about it this whole argument will just go in circles.
Looking at the chapter, this could be somewhat relevant regarding natsu's strength. We saw natsu attack the war god twice. The first one he did nothing, the second one he ended him. If he did use the same power he used against zeref it puts the war god in an entirely knew light. It's not just some creature natsu one shot with a flame punch, its a creature that resisted his full power (not his strongest technique but still the most power he could get out of said technique) even considering evidence suggests natsu's base form far exceeds pre time skip gildarts. The yakuma war gods are instantly immensely powerful creatures that would even be a threat maybe even to springans...
