There are many, many undead in this world.
Su Ziyu had just encountered one of them, and it was a relatively weak one at that. He definitely doesn't underestimate the power of the undead, because so many Lords of Cinder have been burned on the First Flame throughout history, proving many things. Since the start of the Fire Transmission Ceremony, the undead have never failed, and Su Ziyu wouldn't underestimate their power just because he easily defeated one undead.
Chaos Witch, White Dragon Heath, Tomb King Nite, all those strong beings who gained the power of the King's soul during the Age of First Fire, were burned on the First Flame by the undead all the same?
They've even burned the Divine Race of this world many times.
