Relics are special items that make shadow enemies stronger just by being in your backpack. Also, unlike most other items, you cannot discard these items from your backpack.
Shadow enemies are endless monsters that appear in the Eastern Forest and the Forge's Cellar. They grant no experience, no Coin and no potions. In the absence of relics, are extremely easy to combat or avoid.
At the end of the Eastern Forest and in a random location of the Forge's Cellar, you can find an altar. If you bring a spell relic to the altar in Eastern Forest, you will be rewarded with increased spell damage. If you bring an item relic to the altar in the Forge's Cellar, it will restock an item in the Forge's shop.
There is someone in the eastern room of the temple that will give you spell relics if you request. They also will remove relics from your inventory for a small fee. You will need to have learned spells before you can gain access to the eastern room of the temple. Each spell relic will upgrade the damage of one specific spell; pick which one you want to upgrade when you request the relic. Each time you successfully upgrade the damage of a spell, relics for the same spell will be more powerful, that is, they will make shadow monsters more and more powerful.
There is someone in the Forge that will give you item relics if you request. They also will remove relics from your inventory for a small fee. Each item relic will restock the matching item type in Forge's store (weapon, shield or set of armor.) Each time an item is restocked, the next item has a higher item level. While you can get the store to restock by merely purchasing an item, restocking through the relic increases the item level several times more.