A bit late to the party, but - the origins of the Frog cult were the reason I got so into Jennell Jacquays' (all possible spelling mistakes, I made with that one) "Caverns of Thracia". The d20 version contains an optional boss that IIRC goes by the name of "The Immortal King", and that supposedly created the civilization that in turn built the lower levels of the dungeon. (The subterranean caves, IIRC.)
IMC, the Caverns of Thracia were located in the Duchy of Ten - and they were basically the next stop after the TotF dungeon, which we played, IIRC, as per Supplement II, not as per DA2. I never fleshed the story out much, but because, for campaigning purposes, it's such an easy pair, with two fairly big modules laying things out in a pre-prepped fashion, maybe going back there someday - for somebody else than me - might be worth a thought.
I also like the idea of bringing Henry-Rider-Haggard-ish primordial beings into BM is something that I like, because Castle BM always suffered from the "Grayskull Paradox" - like "Castle Grayskull" from "Masters of the Universe", it's the center of the setting's narrative, but we never learn quite why. So, supplementing the pre-history/"founding era" of the present-day setting this way is something that pays off on the long run, because it gives you a background to work with beyond what the setting is already supplied with - which is not much, IIRC. (Now, I never really got into Mystara's version of BM, and I think the *why* of the setting is explained way better than just in Arneson's version.)
|