How can Venat be the previous Azem?

[Theozilla]

There's also the fact that we have missing pieces of the puzzle here that may have to do with Azem. We have not dealt with the "roar from within the earth" that caused the transformations into Terminus back in the pre-Sundering days...a sound that was confirmed to be heard at Radz-at-Han when the transformations began. I have a feeling that's going to deal with our next big journey, and possibly with what Azem was doing. Zodiark dealt with the imbalance of aether, Hydaelyn sealed him to prevent more sacrifices and so the imbalance wouldn't return...so what caused the original imbalance, where did it come from?

That has been answered already in Endwalker, the Sound/roar was all but literally stated to be due to Meteion’s Song of Oblivion.