Fusion is an option on the main menu that allows you to sacrifice other heroes to gain EXP and levels with the cost of gold. Depending on the level and rarity of the card that is being sacrificed will provide different amounts of EXP gained. The amount of gold required to fuse depends on the level of the hero receiving the fused cards at the ratio of 100 Gold per level per card. If the cards being fused have the same Initiative Skill as the selected hero, you have a chance at leveling that skill. You can increase that chance by fusing multiple cards with the same skill. To achieve Skill Level Up: 100% - multiple cards with the same Initiative skill need to be fused at once until 100% is achieved. You are able to see this percentage on the Upgrade interface at the bottom.
The ADD button automatically selects Common or Uncommon cards that can NOT evolve, so you do not have to worry about the system auto-selecting important cards like protagonists.
Each rarity will provide a different % of skill increase, typically the higher the rarity the higher the % rate. You are also able to combine different rarities to achieve the desired % rate to level up your hero's skill.
Note: Initiative skill levels reset when a Hero is evolved from normal to Elite or when evolved to a higher Rarity, they do not reset when evolved between Elite to +1, +2, or +3.
In addition, Epic Heroes provides specific cards for fusing purposes that give a substantial amount of EXP. These cards are Sunflower, Pumpkin and Pumpkin Prince. They can be obtained in the event dungeon: Pumpkin Patch on their corresponding days of availability. You also have the option to evolve these cards to reduce the cost of fusing. Sunflowers can be evolved into a Pumpkin, and Pumpkins can be evolved into a Pumpkin Prince.