Amulet of the War Mage

When you cast Mage Armor, the spell functions normally,  (AC = 13 + your Dexterity modifier).
In addition, the amulet entwines its necromantic essence with your magic - for as long as Mage Armor is active on you, you gain resistance to any one energetic damage type, chosen at the time of the casting.  (you may change this damage type with each casting of Mage Armor - No physical damage resistances allowed)

A necklace crafted from dark silver, holding a small, cracked shard of black crystal that resembles a dried, hollowed piece of bone.  Under light, the crystal pulses with a deep violet glow, as if something inside were beating with a slow, lifeless rhythm.  

Notes: Resistance: Choose a Damage Resistance, Arcane Spellcaster, Buff