So is Erebus actually going to die, or did his master prepare something for him to heal himself?

There was nothing prepared for him to heal himself. Both of them dying was the one scenario Tsa'rahlitzek did not prepare for, because of this Erebus is not just dead but very dead, having used up his undeath as well so he could disappear off into the sunset - thus providing something for ne'erdowells to be nervous of for decades to come.

What will the focus of the next volume be on? Will Alec finally get some time to grow strong?

I'm going to answer both of these questions at once as the answers are one and the same. Yes, Book 3 will finally be Alec's time to shine, and will be much lower stakes than the previous two. In it I intend to properly flesh out the magic system of Reath which has existed as a set of notes since Book 1 but has never really had a good time to be relayed to the audience but Alec and Holly finally getting formal training will be as good an opportunity as I could hope for.

This is my fault entirely for having plots with ticking timers constantly pushing things forwards.

Stolen from its rightful place, this narrative is not meant to be on Amazon; report any sightings.

There will be other hanging plot threads I intend to address, we'll see the growing efforts to free Von Mori, the consequences of Agh'zak breaking one of the aetheric chains binding his people (complete with a genuinely unforgivable pun) and the beginnings of an attempt to resettle the Underreath.

I might not manage all of these but I certainly intend to.

How big a part will the prophecy play in the next book?

Almost none. There will be a few bits and bobs in the background, and certainly a lot of people will be reacting to the prophecy itself. But the prophecy is more intended as a big heavy threat just waiting to happen for now.

What happened/What will happen to Lutan?

Well Lutan's in a spot of bother and is going to face a very difficult choice. Like it or not bardic magic has made his life a hell of a lot easier over the years but he'll no longer be able to pass various magical mind scans etc without losing his position and likely his life as well. He's going to face a choice, either lose the memories and go back to having to fight through mortal means or live in constant danger of discovery and death.

More than that as someone who hates magic, he's going to have to come to terms with the fact he has become the very thing he swore to destroy.

What about Von Mori?

Well her forest is doing everything in its power to free her, but forests aren't exactly huge on mobility so they're going to have to contract the work out. If only they knew someone full of retired heroes who's just love one last moment to shine before they die...