It's the personal armor of centurion Gaius Magnus. It's a full suit of heavy gauge steel similar to Legate Lanius's armor, with a light green and yellow tint, and the Bull of the Caesar's Legion is painted onto the front and the back of the breastplate. The Roman numerals for 87, LXXXVII, are spelled out down the back right of the armor.
This armor has a massive amount of item HP, more than any power armor in the game. It also provides 22 DT, which is equal to T-45d Power Armor.
Even though it has the Caesar's Legion symbol on the front and the back, the armor is not considered faction armor, therefore it can be safely worn in any location without repercussions.
Originally, the Courier could side with the Legion and obtain the armor from a tributary case left by the Legion as a reward, however, this idea was scrapped and the tributary case only exists as an unused object in the GECK.
Behind the scenes
When asked about the armor, Chris Avellone stated:
“We kept hearing people say they wanted something as cool as Lanius' armor, so we decided to give the fans a special version. After all, it is the last add-on.”