The Umbral Skin is a set of plates dropped from the boss Zyr the exiled.
there are 4 sets of plates that will give a different attribute in the 3rd slot.
The blue set will have +3 power regeneration for the 3rd attribute. 5200, 5201, 5202, 5203
The green set will have +3 hull regeneration in the 3rd attribute. 5204, 5205, 5206, 5207
The purple set will have +3 psi regeneration in the 3rd attribute. 5208, 5209, 5210, 5211
The yellow set will have +3 crew regeneration in the 3rd attribute. 5212, 5213, 5214, 5215