Contamination

With the xenos invasion came terrible new weapons; nuclear fire has scourged parts of the world and removed entire extra-dimensional realms for existence. Worse still, it leaves an insidious poison behind - invisible and invasive, radiation destroys its victim from inside out.
The Contaminated condition represents exposure to harmful levels of radiation.
Levels of Contamination: The condition has multiple levels. Every time a creature is afflicted with the Contamination condition, the level raise by one. The effects stack with each level.

Effects per Level:

1st Level: The creature has disadvantage on Constitution saving throws.
2nd Level: The creature's movement speed is reduced by 10 feet.
3rd Level: The creature takes 1d6 radiation damage at the start of each of its turns.
4th Level: The creaturehas disadvantage on Strength saving throws.
5th Level and Above: The radiation damage increases by 1d6 per level beyond the fourth.

Removing Contamination: A creature can make a Constitution saving throw at the end of each of its turns, reducing the condition by one level on success, and two on a critical success. Effects like lesser restoration can also remove one level of contamination.

Contamination in the World

Contamination is greatly feared but poorly understood. Entire areas have been made uninhabitable through the devastation unleashed by xenos nuclear devices, and immortals lain low by the split atom. Still, people adapt. The Way of the Megaton Fist is an monastic order of monks who have turned the poison into a source of power, while the Church of the Radiant Atom worship it as something greater than even the gods.
While the xenos have bound nuclear forces to their whim through technology, magic is able to interact with it in new, unexpected ways. Unfettered by limitations imposed by reality, wizards have managed to create entire new schools of nuclear-powered magic and enchanted items
Radiation has created mutants and aberrations never before seen in the world, warping mundane and magical creatures into fresh horrors. Entire species have been slain, and new ones created from the nuclear furnace.