Regulus is the realm of the modrons, the largest known realm in all of Mechanus. It consists of 64 interlocking cogs arranged in the shape of a pyramid (somehow; information on this is pretty vague); on each cog is a vast city built around a central tower where an Octon rules.
In the center of the realm is the tower of Primus, the Modron Cathedral, and quarters for seven modron armies.
Primus sealed the borders of Regulus after the last Great Modron March in order to give the modrons time to recover from the unexpected chaos of it all.
Sources:
Planescape Campaign Setting
Planes of Law
The Great Modron March
Dragon Magazine #341
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