Mutants

The Demon Lord's power influenced the world, starting as early as a full decade before his escape from his prison. This influence has, amongst other things, given random mutations to people; marking them as future servants should he succeed in his plans.

While there is a lot of variation amongst mutants, there is some consistent patterns. All mutations manifest quickly over the course of a few months, with some of the "lower tier" mutations being mistaken for medical anomalies at first. After fully manifesting, many mutations grow more "extreme" at a very slow but constant rate: i.e Marmar's horns growing slightly bigger each year. The other consistent rule is that only people currently undergoing puberty can be affected by mutations.

Mutation Tiers
Henri Ackerman has come up with a system to classify mutations, with complex rules and scoring to assign them to one of multiple tiers and subtiers, but after much prodding, has simplified it so everyone else can understand it.

Tier 1
The beginning mutations, some have been mistaken for non-supernatural medical anomalies; they may have left doctors baffled but a good portion of them could make it into Ripley's Believe It or Not without attracting too much attention. For the first two years, only Tier 1 mutants were created. Maria St-Marie, Isaac Anderson, and Britney are all tier 1 mutants.

Tier 2
These more advanced, more overtly supernatural mutations are harder to hide, but are less minor than Tier 1 mutations. However, they do not go as far as full-body transformations or new limbs. Tier 2 mutations began appearing after roughly two years Wolfie, and Cleo are from this era.

Tier 3
These more extreme mutations feature full-body physiological changes, or manifestation of completely new limbs. Tier 3 mutations began appearing a further 2 years in. Henri and Nessie are both Tier 3 mutants.

Tier 4
This final tier of mutation is roughly defined as just being just too monstrous to qualify as a Tier 3 mutation, to the point where disguise is impossible. They began rarely appearing two years after the first tier 3 mutants, and slowly became more common over the remaining 4 years. Many members of Camp Mutie are tier 4 mutants.

Tier X
A "purely hypothetical" tier, reserved for any mutations that are both wildly extreme and capable of mass-destruction. A Black Shard can embed itself in a mutant, temporarily turning it into a Tier X mutant at the cost of succumbing to the Demon Lord's control.