The Great Rain of Fire
by Bruce HeardMore about the Great Rain of Fire. Checking with GAZ3/Glantri, DA3 City of the Gods, and Wrath if the Immortals we have the following pieces.
The Beagle landed on Mystara during the Blackmoorian era. Blackmoor at the time was a pre-technology civilization. The starship's crew eventually caused their technology to become commonplace among Blackmoorians. The latter eventually detonated "a device" provoking the Great Rain of Fire in 3000 BC.
If you read the intro box on page two of GAZ3/Glantri, you'll notice a clue about what the Blackmoorian device really was. Sic <<Eons ago, when Blackmoor still was a great empire, visitors came from the stars in a great chariot of fire and landed in the realm of mortal men. Stranded, they soon disappeared from this world, leaving remains of their science. Among these, a huge object producing a deadly glowing energy was buried deep in the rocks under what was to become the City of Glantri. It was a great, wonderful piece of machinery, indeed the very one that enabled visitors to travel among the stars. Yes! Oh, Seeker of Lost Legends! This is the true nature of the Radiance.">>
In other words, the Nucleus of the Spheres is what remains of the Beagle's engines. This quote isn't entirely accurate since what made Blackmoor so powerful *was* the technology acquired from the Beagle's crew (details, details...) Else, the entire event is very clearly described in the adventure book in Wrath of the Immortals (check pages 6-7 in particular).
The Beagle self-destructed, but its engines survived and sunk deep into the rock. Later Blackmoorians did eventually figure out what energy powered the Beagle, but they failed to control it (Mystara's magic properties caused a chain-reaction), provoking the Great Rain of Fire. It is Mystara's magical properties that caused the Beagle's engines to become a true magical artifact when the rest of the ship disintegrated.
Eventually, an intrigued and amused Old One found the remains of the engine and its magical effects, and moved it to its current location, underneath Glantri. Immortals later altered the engine to balance out its impact on the five spheres (thus its name "Nucleus of the Spheres"). As a result the artifact began leeching magic from Mystara. In 1700 BC elves finally caused another explosion, the one that messed up the Broken Lands. This explosion was the result of an "unknown" Blackmoorian device, presumably abandoned in the area.