Once your castle has reached Level 5 the Dragon Cave will become unlocked, and you will be able to enter it to hatch and raise dragons in order to add a fifth member to your party when you prepare for battle.
There are five different types of dragons in Infinity Kingdom and each one is tied to an element – Water, Water, Earth, Wind, and Lightning. Each of these dragons has 5 maturity stages – Egg, Infancy, Adolescence, Adulthood, and Elder.
You will need dragon soul crystals in order to hatch a dragon egg when you reach Level 5 and unlock the Dragon Cave. Dragon soul crystals can be acquired as rewards from world bosses, quests and events. Once the dragon egg has hatched, you will be able to raise and upgrade your dragon by spending gold, which can be obtained from the Coin Mint and Dwellings buildings. You will be able to train a dragon when you see a dragon icon bubble above the Dragon Cave. When your dragon reaches level 5, 15, and 25 you will unlock skills for them. This skill and their effects can be upgraded by spending Dragon Soul Crystals.