What the hell happened 6 millennia ago?

Books, yes. Through their journey of shutting down that Portal deep within the Castle, near the new Lan Zhe. They've found 4 books that are unmistakably written in the Calidum Lutum language and the 5 other books represent the Ignite language although still a blur.

It is hard to make anything out of the Ignite books but the ones written in the Calidum Lutum language are a firm 70% understandable.

The Calidum Lutum language hasn't developed through time thanking the Grand fact that the entire race has been crammed in one place for 6 millennia.

It would take too long to state point by point what all 4 of the books are going over, but it would be acceptable to quote a handful of paragraphs representing the key points.

"The Beast, it needs feeding. It demands bodies, what will we do? We have no more wrong doers to sacrifice no longer!" This is a quote from the oldest seeming book that has a lot of its pages ripped out.