Perhaps one of the most mysterious energies. The Mana represents the key to unlock rifts from the Planes to dimensions, allowing to take energy from them.
Generally Mana can be obtained and applied via specialized organs or devices, which by high chance, are gonna be made by the Deus. More about Mana usage in Magic see here.
Nature
The true nature of Mana lies behind comprehension for non-Deus. Supposedly the Mana represents kind of a glue energy connecting planes together into dimensions.