Honestly, I was pretty surprised that this was mentioned at all. I think, Larian is preparing something very exciting, if they've been digging THIS out from the lorebox.
The game is a "sequel" to the
Baldur's Gate: Descent to Avernus module where the Blood War is a big element of the story.
If you talk to Zevlor after entering the grove (I think you need to talk to him in his cave for this), he will mention The Descent...and you'll get an info dump for it.
