At the cost of mobility, a warforged character’s body can be crafted with a layer of adamantine that provides formidable protective armor and some damage reduction.
Prerequisites: Warforged, 1st level only.
Benefit: Your armor bonus is increased to +8 and you gain damage reduction 2/adamantine. However, your base land speed is reduced to 20 feet, and you are considered to be wearing heavy armor. You have a +1 maximum Dexterity bonus to AC, a –5 penalty on all skill checks that armor check penalties apply to (Balance, Climb, Escape Artist, Hide, Jump, Move Silently, Sleight of Hand, Swim, and Tumble), and an arcane spell failure chance of 35%.
Normal: Without this feat, your warforged character has an armor bonus of +2.
Special: Unlike most feats, this feat must be taken at 1st level, during character creation. Warforged druids who take this feat cannot cast druid spells or use any of the druid’s supernatural or spell-like class features. Warforged characters with this feat do not gain the benefit of any class feature prohibited to a character wearing heavy armor.
Source: Eberron Campaign Setting