Soul is a resource that is regenerated every hour. You gain a fixed amount of soul per hour depending on how high of a level your Dungeon Heart is. At a level 10 Dungeon Heart you will gain 1000 soul per hour. There are 3 ways to refill your soul completely. The first is to upgrade your Living Area, the second is to upgrade your Dungeon Heart and the third is to buy and use a Soul Liquid.
You can also harvest 1000 Soul Crystals every 3 hours. Once you collect your crystals you will have to wait 3 hours for them to recharge. If you do not collect them at the end of those 3 hours they will wait until you collect them before beginning their growth again.
If you are the winner of a gold mine then you will receive a soul refill.
Another way to gain soul is to participate in PvP and attack another player to loot some of their soul.
You receive a 10 soul reward for participating in chat.