An option is to instead have a "mantle" that gets passed down through the descendants. When the current holder dies (or gets too old or something), it gets passed automatically to another. What ...

Depending on the axial tilt of your planet, the poles might be an option. At near 0° tilt, the poles could easily be warmer than the night-side equator. With 0° tilt, someone standing at either pole ...

Male Cambions are sterile. Cambion eggs are incompatible with humans, but compatible with demons. IRL, egg and sperm have a "lock and key" system to prevent cross species fertilization (to an extent, ...

Earth was worried about the colony declaring independence, so the high tech was programmed to phone home periodically. If the colony rebelled, Earth could send a kill signal (or just stop responding), ...

Another option is that wood is somewhat porous, while metal isn't (as much). The regeneration can work around and eventually push out a metal object, but enough body material gets absorbed by the wood ...

Willpower. Humans controlled magic. They still have some residual control over the pollution. Less humans are mutated because humans don't want to be mutated.

Their deity drains mana from the fae realm, leaving the mortal realm as their only source, and they can't survive there very long. Only beings born and raised in the mortal realm have mana to drain. ...

Another option is to have the mineral be extruded from (directly or after some geologic processing) living organisms, that evolved in one area and can't spread due to surrounding conditions being ...