Rookie
Enemies deal 75% damage and you gain 70 armor. The threat clock climbs at the slowest rate.
Difficulties
Hero is the default. The names on the threat bar are the same on every difficulty. The clock is what changes.
Hero
The standard run. No extra armor, and enemies deal full damage. This is the default.
Legend
Same damage as Hero, but the threat clock climbs faster, so elites and levels arrive sooner.
Threat bar
As minutes pass, and again after each stage, the run moves through Calm, Rising, Fierce, Brutal, Savage, Nightmare, Cataclysm, Apocalypse, BEYOND. Elites get meaner as that bar climbs. A single hit cannot empty a full health bar by itself.