When you get him to half health, he will transform.
Flaming Two-handed sword
When Serien transforms into his second form, things get worse. He now has a stronger attack but is still about as hardy as he was before.
Grapple - He grabs and inflicts heavy damage
Flame breath - A Red cone appears in front of him. He will target and breath fire on the people within that area.
Slash - Attacks with his right hand using the blades.
Flame Slash - A Red cone appears in front of him. He will swing his right arm favoring the torch on it.
Smash - Charge then brings down both club on target. Can be interrupted.