Synthesis is a feature that allows players to upgrade Heroshards, Runes, and Runeshards to get 1 NFT hero, a higher level rune, and a rune level 1 respectively. Before synthesis, players must also use $ES as a fee. Furthermore, not every synthesis will be successful; some may fail. On the other hand, every time a player fails to synthesis, they will have a better chance of succeeding the following time. Furthermore, the amount of material you used for synthesis will increase following each failure.
The graphic below illustrates the number of materials to be consumed, the number of $ES to be spent, the success rate of each count (The number of times that players have failed to synthesis) and the returning rewards.