The FSS Beagle was a Space Ship from the Galactic Federation that got
lost during a miss-jump and ended up in Mystara Space. Due to technical
problems, it crashed on the Planet. It landed in what later became
known as the Valley of the Ancients, south of Blackmoor. The Ship
itself was referred to as the City of the Gods. Some sources say
Mystaras magical energies interfered with the advanced technology of
the Beagle. However, I become more and more inclined to believe that
it was Immortal interference that led to the Spaceships crash. After
all, the crash led to a chain of events which eventually caused the
destruction of the Blackmoor civilisation.
Although the federation had a Prime Directive which forbid interaction
with barbarian worlds, these regulations were not followed and contact
between the natives and the FSS Beagle led to among other things a
meteoric rise in technology for Blackmoor which in turn allowed the
Blackmoor civilisation to slowly become the dominate power of the
entire planet. However, by combining magic and technology, something
went wrong. The result was a disasterous cataclysm which removed
Blackmoor from the face of Mystara and changed the world dramatically.
The events were later referred to as the Rain of Fire. (The Mystaros
Timeline provides an alternate explaination to the Rain of Fire).
By the time of the Great Rain of Fire, the FSS Beagle was already long
gone. During this whole time, most of the Beagle Crew had been in
suspended animation. However, Ogdoban Treel, a communications officer
woke up during a routine check (200 years after the crash)and found
that several crew members including Captain Bork Riesling were missing. His motivations are unclear to me, but he caused the Sleeping Tanks of the other crew members to malfunction, killing them and contacted the natives beginning to sell off technology. However, First Assistant Engineer Rheddrian Benekander had survived. Having tinkered with his sleeping tank, he was not killed along with the rest of the crew, but was instead awakened, finding to his surprise that the Ships core had become unstabilized by Treelís tinkering. Rheddrian realized that the ship would be destroyed but decided to sacrifice himself by turning the imminent explosion into an implosion. Both Rhedrian and Treel were aboard the ship when the reactor imploded. Rheddrian and Treels fates are detailed further in WotI.
However, the Ships Reactor caught the interest of an Old One who turned it
into the Nucleus of the Spheres and placed it below the land that now
is called Glantri.