Who exactly is 353?

[Eight Fingers] had been occupied by an unknown intelligent and powerful magical creature, and even the key figures were enslaved.

This incident had never occurred in the original story.

At this point in the original story, [Blue Rose] and [Eight Fingers] had their first conflict, which led to a series of subsequent events and eventually involved the people of Nazarick, causing greater chaos. The sudden invasion of a powerful magical creature taking over [Eight Fingers] was an event that had never happened in the original story.

No, it wasn't accurate to say that such an event had never occurred.

In the original story, at the end of this incident, [Eight Fingers], which controlled the Kingdom Underground World, was indeed controlled by Nazarick, but that happened after the incident concluded, not at the beginning.