Hayato wasn't entirely bluffing.
Orochimaru's chakra natures were fire, wind, lightning, earth, water, yin, and yang. He or she knew a little of every type of ninjutsu and had mastered quite a few, possessing the strength to compete with Sarutobi Hiruzen for the title of "Shinobi World Professor."
But—
Aside from Impure World Reincarnation, the techniques Orochimaru used most often and best were still snake-related, including the Eight Branches Technique that could go head-to-head with Itachi Uchiha's Susanoo—though it could not win.
The essence of the Eight Branches Technique was the power of the White Snake, while its outward form was a massive eight-headed serpent monster. Though Yamata no Orochi was a transformation technique, it still had flesh and blood and was a close-combat type.
As long as it had flesh and blood and fought at close range, it was nothing but a little brother before Tyrant King's Moon—Night Guy smiled without speaking, as if to say, Whatever makes you happy.
Even though Hayato's Tyrant King's Moon was still under development, if Orochimaru dared to use the Eight Branches Technique, Hayato dared to meet it head-on.
That meant Orochimaru's only trump card left was Impure World Reincarnation. But Orochimaru's Impure World Reincarnation had not yet been perfected, and he himself was in poor condition. Its power was easy to imagine.
All in all, Orochimaru really had no good way to deal with Hayato, which gave Hayato room to brag. Bragging was free, and it could irritate Orochimaru too, so why not?
Not entirely bluffing meant there was still some bluffing involved. At the very least, Hayato did not think he had a way to completely kill Orochimaru. One Cursed Seal was already troublesome enough; besides, who was to say Orochimaru did not have other hidden methods of immortality?
Hayato believed