Each time you fail to enhance an item, you will see "Agris Essence" start to fill up in the "Ancient Anvil", which, when activated, will grant 100% enhancement success rate for said item.

To put it in plainer terms, the Ancient Anvil is a "guaranteed enhancement" system of sorts.

 

For example, let's say you fail to enhance your TET (IV) Deboreka Necklace and fill up the anvil with 1 Agris Essence. Fail enough attempts and with enough Agris Essence, you will attain 100% enhancement success rate for the TET (IV) Deboreka Necklace. And, for example, even if you fail enhancement and your TET (IV) Deboreka drops to TRI (III) Deboreka, if you bring it back to TET (IV), you will see the same amount of Agris Essence previously stored preserved.

 

Unlike accessories, weapons, and defense gear (such as Blackstar, Slumbering Origin, etc.), store Agris Essence by set groups. In other words, you will retain the same amount of Agris Essence stored when attempting to switch enhancement from Blackstar Longsword to Blackstar Greatsword, as long as the weapons are at the identical enhancement level.