Bone Mail
April 4, 2017
15:24
Found in Drakenvale during Act 2.  You need to actively search a skeleton on the ground, I believe, in order to find it - it's not in any visible chest or anything.  More can be stolen from an enemy in the Cleft of Dimensions, though obviously that's very late in the game.
Bone Mail is the single highest DEF piece of armor in the game (by almost half over the Genji Armor!), wearable by any class capable of wearing Medium or Heavy armor.  Its DEF is so high that it actually -nullifies- many enemy attacks, or reduces them to near-nonexistent damage.  It also possesses several other qualities:
Absorbs Poison damage
Halves Ice damage
Weakness to Holy/Fire damage
Several status immunities: Death/Poison/Darkness/Old/Confuse/Berserk/Regen
Sounds amazing, right?  Well, it is, but there's a catch.  Bone Mail causes the wearer to become Undead.  All effects that would heal Bone Mail's wearer - curative items, white magic, etc. - will instead deal damage, because the wearer is now an "undead".  Beware - Elixirs (and other "full heal") effects will set you to critical HP instantly, and Phoenix Downs or similar will kill you outright!  You also can't be revived mid-battle!
Though, there are ways around this.  First, Drain effects always favour Undead targets - whether cast by enemies or allies - so, for instance, a Knight wielding the Drain Sword or a Blue Mage using Vampire can recover HP.  Second, all of the following spells and weapons will FULLY HEAL an Undead: Death, Roulette, Level 5 Death, Phantom, Death Potion, Assassin Dagger and Death Sickle.  Third, any character with a Flame Ring/Flame Shield/Ice Shield can be hit by said element, and will be healed (as they absorb the damage).  You can also use poison-element attacks (namely Black Mage's Bio).
Bone Mail won't serve you well in battles against bosses that are going to go on long enough you need to heal. It will serve you damn well in many random encounters (You can always heal out of battle!) or against bosses that do purely physical damage though.  Pick and choose when to use this armor and it'll be one of the most amazing pieces of equipment out there.