Endurance is a defensive mechanic that allows you to take less damage while below a certain health value.
While your health is less than your Endurance Threshold, you will take less damage equal to your Endurance value. This reduction applies to health, but does not apply to ward.
Endurance has a cap of 60% less damage taken (multiplicative with other modifiers), while Endurance Threshold does not have a cap.
Endurance applies to any damage taken below your Endurance Threshold, even if the instance of damage applied while you were above it. For example, if you take a hit of 100 damage, but 50 of that damage was below your Endurance Threshold, the last 50 damage will have Endurance applied to it.
All characters start with 20% Endurance and have an Endurance Threshold equal to 20% of their maximum health.
Endurance mitigates both hits and damage over time.