Diamond Beast
Nothing in existence was perfect, even the oh-so-powerful ancient beings from the legends had their own weaknesses and shortcomings as in the end all of them have fallen and succumbed to either the clutch of time or someone else's powers.
Hence, if those beings from the Ancient Era who was capable of demolishing planets and creating separate domains with a single command or flick of their fingers, how would a 'weak' Diamond Beast that didn't even exist for more than a hundred years have no weakness?
Nash and the rest it was simply impossible for such a beast not to have a weakness. Even if it was a small one, there definitely was one despite not seeing it at a first glance.
"And what is this Diamond Beats' weakness?" Nash asked, curious to know such a seemingly unkillable beast's drawback.