Also known as "Sacred Treasure", these items are available in sets of six and offer various R-rarity rewards.
You must collect all six pieces of a set in order to unlock its reward. While four pieces are obtainable from the campaign, the remaining two must be taken from other players via duels.
Each set can be completed up to three times — the first and third time offering you a unique card, the second time always offering a Metal Slime. When a set is completed, you are prompted on whether or not you wish to claim your reward right away. If you select "no", you can claim it later via Menu > Collection > Sacred relic.