How can Venat be the previous Azem?

[Iscah]

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.

That depends whether the writers intend for it to be a separate phenomenon or a strange effect of Meteion's call.

The primary mystery with Azem right now is how they knew to tell Themis to expect us.

That may get entirely resolved within the raid story, and all plotlines regarding the ancients wrapped up within this patch cycle, but we'll have to wait and see.