The Heart of Power gets unlocked after joining a Guild. It allows you to access the Forge, to upgrade your Heroes Glyphs, and the Gift of the Elements.
Forge[]
In the Forge Heroes can upgrade their Glyphs by using Enchantment Runes and Gold or by spending Emeralds after joining a Guild.
Gift of the Elements[]
The Gift of the Elements grants your Heroes a passive primary stats bonus (Intelligence, Agility, and Strength) through investing Sparks of Power and Gold.
The main hero stat gets double the benefit. For example, Astaroth's main hero stat is Strength, it gets raised by 4 at the first level while his Agility and Intelligence are raised by 2. Increasing the primary stats will also increase certain secondary stats such as Magic Attack, Magic Defense, Physical Attack, Health and Armor (see Stats).