The Lesser Arcanum of Constitution can be obtained by completing the quest  , obtained from in the Burning Steppes. The other materials required to complete that quest are listed below. Once the arcanum has been obtained, it can be applied to any one leg or head item, giving the item an additional 100 health. The arcanum is consumed when it is applied.
As of Patch 3.1.0 the benefits of this enhancement do not become active until level 50. Although the enchantment is not removed from an existing item, you will not receive the benefit until you meet the new level requirements. This item was popular amongst low-level twinks, but they will now see a new in-game tool tip "Enchantment Requires Level 50" in red font (indicating the enhancement is not active) if their character is below the required level.