Unlocked in the Crystallite Deposit after defeating the FIRST 'Chosen By Xi' Enemy, which occurs very early in this map. You also need to have researched the Spartan Armor before you can start researching this armor. This is the best armor in game as of Black Legion update (even better than Black Legion Armor, at least for PvE).
Here are the numbers (with a "General of Army" Matryoshka):
| Armor | Overall HP | HP restored with each U3 Medkit |
HP restored with 1 U3 Medkit |
HP restored with 2 U3 Medkits |
HP restored with 3 U3 Medkits |
|---|---|---|---|---|---|
| Black Legion | 4300 | 2150 | 6450 | 8600 | 10750 |
| Chosen by Xi | 3800+1100 = 4900 | 1900 | 6800 | 8700 | 10600 |
Without healing, the overall HP ("normal" 3800, plus around 1100 after reanimation) of the "Chosen" armor outperforms the 4300 HP of "BL" armor. Only in a battle with at least 3 U3 medkits used, will the BL armor gain advantage over the "Chosen" armor. (NB: these battles are extremely rare.) However, BL armor still lacks the "Chosen's" unique capability to absorb an unlimited blast of damage prior to reanimation - especially useful if a bomber (or even the bomber queen) manages to get close enough to you.