Two heroes can be combined to create a better hero of higher rarity. To do this, both two heroes must be at their maximum experience and upgrade level. The experience level is increased when that hero fights in a level. The upgrade level is increased by upgrading the hero, which is done by sacrificing other heroes to use as fodder. If you plan on combining a specific hero, swap them out after they reach their maximum experience level for someone else who does not have enough experience. This will ensure that your team is fully leveled and ready to combine when needed.