User blog comment:B214/Question and Answers 6/@comment-2607:FB90:2B65:F6D:849D:C9C8:262F:BFD2-20200129195814/@comment-10330822-20200130021444

Dragon slaying is usually a bonus put on Holy Swords. Ascalon is one such example. Where its a holy sword first and a dragon slaying sword second. Ascalon II is implied to be more dragon sword than holy sword, so its like the opposite.

However, Gram was to demon swords what Caliburn and Excalibur are to holy swords. The only thing is Caliburn and Excalibur do not have the dragon slaying advantage.

Kiba was initially creating a dragon slayer sword from his Sword Birth sacred gear, which is the parent of Holy Demonic Sword. Theoretically, Kiba may have completed it since he wanted it to fight Sigfried. And, the qualities of the Holy Demonic Swords can be changed unlike the holy swords he creates with Blade Blacksmith.

Now, the problem with Kiba's battle with Issei is that Kiba needed to use speed to help compensation for Issei's far superior power. Hence why he used Blade Blacksmith/Glory Drag Trooper in the fight rather than Sword Birth/Sword of the Betrayer.

And that in lies the problem, Kiba couldn't use Holy Demonic Sword in the battle because he needed Glory Drag Trooper for the speed boost.

Hypothetically, Kiba would be much more dangerous if the two sacred gear fused, and became a "Blade Birth or Sword Blacksmith" sort of thing.