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Relics are equippable items that you acquire or find as you progress through the game. It costs 10,000 scoria to equip a relic and you can only have one equipped at a time.

There are six total relics. You start with one equipped (which I believe has no effect) and must find and equip the five others for the achievement to unlock.

Ukkoto's Guile. Gefn's Blessing, and Umbral Veil are rewards for defeating the 1st, 3rd, and 4th bosses and therefore cannot be missed.

Guardian's Pact is given by Kol the Wildman who can be found in a room under the giant Diasora in the Whispers area. Look for this wooden scaffolding on the side of the huge structure that faces the Seat of the Matriarch Ritual Stone. Do an assisted climb with your AI partner to get on a stone platform around the corner and then jump onto the scaffolding. Climb the nearby ladder and follow a linear path around and through the building. At the top, you will see an unmarked objective marker over Kol the Wildman- interact with him to receive the relic (and unlock an achievement).

Orli's Fuse can be found on top of a large wooden spider structure in the Loom which is right by the Ritual Stone. You must use spear teleports to get to the top of the structure in order to get the relic.

After having collected all five relics, you need to equip each one at Amara's workbench and the achievement will unlock.

Once you defeat the and progress until the next relic fast travel point, you can return to the vagrants nest and purchase the relics from Amaras crafting table. There are 5 relics in addition to the one you start with and it will cost 50000 Scoria to purchase them all.

The relics are:
Hearth: default relic given
Ukkoto's Guile: obtained by defeating Ukkoto (not missable)
Guardian's Pact
Gefn's Blessing
Orli's Fuse
Umbral Veil

This requires you to obtain 6 relics, 5 of which you obtain by playing the story. With the exception of Orli's Fuse & Guardian's Pact which are covered in this video: