What's up with that first boss form?

The strange appearance of the first boss form made me think of something we would normally see inside Sestren, but not in the normal surface world. This, and the strange water-effects when Abadd goes through the clouds, made me come up with this question: did we ever leave Sestren? It wouldn't make much sense if we were still in Sestren though, so there's probably another explanation. Another possibility is that we see the influence of the Sestren system on the "reality" of the surface world itself. Abadd could have been diving in and out of Sestren during the battle. The water effect could illustrate the very fabric of time and space being disturbed as Abadd flies through it.

"The dreamless sleep of eons...Emptiness...That is... true fear." What did Abadd mean by that?

This was probably a reference to the time between the Fall of the Ancient Age and the present time where Abadd was waiting to be awakened. It seems Abadd has some bad memories of this long period of time. I'm not sure why he brought that up at that particular moment though. Abadd feared he would remain alone even after his reactivation, so perhaps he thought the emptiness he experienced before would just remain now that his masters were gone. He wanted to create a new race of drones to prevent this.