If an enemy strikes the caster, their movement is slowed by 30% for 5 sec.
Armor Spells A Mage can only have one Armor spell active at a time.
Anyone can hit a mage. Once. — Morganis Blackvein
Frost Armor is a core mage ability learned at level 54. It grants a self buff to spell haste and slows melee attackers.
The slow effect means never having to fight aggressive mobs you'd rather run past, assuming they don't resist every time. On foot, a chasing creature will get in 2 or 3 hits (some things run faster and swing faster than others) before you outpace them; mounted characters will be hit a single time before getting away.
Patch 1.8.0 (10-Oct-2005): Fixed a bug where the Chilled effect caused by Ice Armor/Frost Armor was not removed properly when shapeshifting.
Patch 1.6.0 (12-Jul-2005): Targets that are immune to movement slowing effects will no longer be affected by the Chill's movement slowing effect. Targets will still have their melee attack speed reduced.