User blog comment:B214/Volume 23 discussion blog/@comment-77.188.244.180-20170422145726/@comment-77.188.244.180-20170422170014

Flame Blaze: Following his training with Tannin, Issei learned how to use Dragon Flames through Boosted Gear. He first inhale an amount of air to fill his lungs, then uses his demonic powers to produce a small flame in within his stomach and use Boosted Gear's ability to enhance it. While the air mixed the enhanced fire within his stomach, he breath out a large stream of dragon flames from his mouth toward his enemies. He's also shown to widen the range of his dragon flames that can engulf the whole sky as shown his battle against Cao Cao in Volume 12. Because of the training, Issei has also developed a high degree of resistance to fire attacks, so that only the most powerful flames, such as those of a Dragon or the Phenex Clan, can injure him.

Blazing Inferno of Scorching Flames: An ability that was sealed by the God of the Bible which Issei managed to unlock upon using his DxD form for the second time in Volume 21. Issei releases a powerful breath of fire which can incinerate anything, flames that never go out once ignited as they continue to burn even after being reduced to ashes and can probably burn the soul, until nothing remains. The only existence that was known to be immune to the flames was Albion, Ophis and Great Red.