Shadow of the Tomb Raider

Vestige outfits (ie, ancient armor pieces that provide unique buffs) are obtained via looting sarcophagi at the end of Crypts and completing certain side missions. These outfits can also be purchased from merchants in Kuwaq Yaku, Paititi, and the Mission of San Juan. 

There are a total of ten vestige outfits in the game. Regardless of how they are obtained, you must first restore them with crafting materials at base camps before they can be equipped. Each outfit has two parts: upper body and lower body. Their resource requirement can be lowered if you unlock the Anole's Needle skill in the Scavenger tree.

For the "To the Nines" achievement, you must restore and equip a matching vestige outfit. The very first set you can obtain is the Eveningstar outfit. Simply complete the two Crypts in the Peruvian Jungle (the first region of the game after the prologue in Mexico) and then, once you have a sufficient amount of materials, craft both outfits pieces at a base camp and equip them.