Lethargy

Lethargy (monster debuff)
Lethargy is a temporary debuff. Observed effects:
 * -1 to all Saving Throws.
 * Non-bosses also move and attack 5% slower.
 * Works on creatures immune to cold.
 * This effect stacks up to 5 times.
 * All stacks are removed when the timer expires.

Sources of Lethargy:
 * Frostbite (enchantment)
 * Improved Elemental Arrows: Frost
 * Water Savant enhancements

Lethargy (player penalty)
When Blood Rage expires, it turns into lethargy which lowers your melee and ranged attack speed by 25%. This fatigue lasts 12 seconds.