Mechanus

On Mechanus, law is reflected in a realm of clockwork gears, all interlocked and turning according to their measure. The cogs seem to be engaged in a calculation so vast that no deity can fathom its purpose. Mechanus embodies absolute order, and its influence can be felt on those who spend time here.
Modrons are the primary inhabitants of Mechanus. The plane is also home to the creator of the modrons: a godlike being called Primus.

Optional Rule: Law of Averages

While on Mechanus, creatures always use the average damage result for attacks and spells. For example, an attack that normally deals 1d10 + 5 damage always deals 10 damage on Mechanus.

Optional Rule: Imposing Order

At the end of each long rest taken on this plane, a visitor that isn’t lawful neutral must make a DC 10 Wisdom saving throw. On a failed save, the creature’s alignment changes to lawful neutral. The creature’s alignment reverts to normal after one day spent on a plane other than Mechanus. Casting the dispel evil and good spell on the creature also restores its original alignment.

Type

Dimensional plane