Quote Originally Posted by PairO'Dice Lost View Post
Note that the Spawning Stone being a part of Mechanus and/or creation of Primus is a 5e retcon.
Calling it a piece of Mechanus may have been an exaggeration on my part. The 5e MM says that Primus created "a gigantic, geometrically complex stone imbued with the power of law", which I imagine would make visitors think of Mechanus until they come across a stream of chaos-stuff or a bunch of slaadi.

Originally, the Spawning Stone was simply a natural feature of the slaad life cycle, which was later altered by the Slaad Lords (all of which were unique beings of great power who came in a wide variety of forms) to ensure that no slaad would ever arise thereafter who could be as powerful as they were and could challenge them for dominance.
I think the two backstories could be reconciled to an extent, with the current form of the Spawning Stone being the result of a pact between the Slaad Lords and Primus.

And now I am imagining a bunch of things about the Spawning Stone:
  • Contrary to what the MM says, there is one last modron enclave in Limbo. Deep under the surface of the Spawning Stone are chambers guarded by constructs that specialize in extracting the control gem from a slaad's body, as well as by the many slaadi they have thus enslaved.
  • The Spawning Stone is a major stage of the Great Modron March, which not only involves many low-level modrons but also high-level ones, inevitables (also linked to Primus in 5e), and War-of-the-Worlds tripods (just because).
  • Further into the depths of the Spawning Stone are refineries where chaos-stuff is permanently transformed into elemental power, and used to maintain the balance of the Elemental Planes. This level is mainly guarded by elemental myrmidons of modron design.