What is secret dragon slayer art?

Question

In fairy tail, during the fight between Natsu and Rogue and Sting in the Grand magic games, Natsu finally uses an attack called Crimson lotus exploding flame edge.
He says that it is a secret dragon slayer art. So, what is so secret about this attack?

Answer 1

I can guess that they are called as secret because only Natsu can use those secret attack , As shown here attack like Dragon Roar are pretty similar to other dragon slayer and those secret attack are very powerful comparing to other attack.

Here you can see , Gajeel also have Dragon Slayer's Secret Art like Karma Demon: Iron God Sword. And Wendy also, like Shattering Light: Sky Drill and Milky Way

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As you mention, while fighting with Rogue and Sting ,Natsu was playing with them and when he became serious he use his secret attack which is one of the important attack for a Dragon slayer. Which are very powerful, even Laxus and Gildarts had hard time handling that attack.