Deities, more commonly referred to as gods, are beings of immense magical power who operate on a level far above most magicians and magical creatures. The Old Gods are the primordial entities that are responsible for the creation of the universe, magic, life and reality itself, as well as the 'parents' of the modern deities of mythology. Alice stated that to the modern gods of myth, humans are like toys, but to the Old Gods, they're like cells in a body.
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The gods are known to be the creators of many worlds in virtually all of the Multiverse. Children of beings known as the Old Gods, they hold immense power that is possibly surpassed only by the Wellspring, the source of magic itself. They are a very ancient race that play a role in shaping the history of Earth as evidence of them appearing in myths. It appears that they can take an active part in human lives and affairs however, most choose not to.
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Deities come in many forms, as diverse as the humans who worship them. Those seen thus far appear to possess a physical form. However, Reynard the Fox has also shown the capability of possessing the bodies of others and work through them, while only briefly showing his true bestial self. He was also able to briefly take the form of his mother to trick hedge witches.
The gods are characterized by their immense durability, knowledge and advanced magical ability. They appear to be virtually immortal and can only be killed by extreme means such as by other gods and powerful magic. The power and complexity of magic a deity can perform appear to be nearly limitless being capable things such as flipping planets over and creating and destroying entire worlds with little effort. Umber, with no assistance, was capable of creating a pocket world of his own design.
Gods can conceive with mortal beings, their offspring are known as demigods and possess great powers. One such demigod was said to possess "more magic coming off him than every Magician on Earth combined".
Gods possess a "god seed" which can be transferred to mortals. Without it, they are left powerless and without magic save for their immortality. Human turned deities can grow their seed through acts of virtue such as answering prayers, performing miracles and granting favors. A human given the seed can become full gods or goddesses if they grow it to completion. The conversion between Reynard and Julia implies that the seed of born gods are also affected by the virtue of their deeds.
The biological essence of a deity has a tremendous affect on magicians. Consuming or otherwise having a god essence within a magician imbues them with power greater than a Master Magician, mentioned by Bigby to be almost god-like. This allows them to touch objects of immense power, such as the enchanted Leo Blade and the key to the Wellspring, and cast spells of devastating magnitude. Being under the essence also grants them temporary immortality, allowing them to resurrect after being killed.
The effects of the boost wear off after an unspecified amount of time. It should be stated that the discrepancy between the boost can be quite large. Alice's boost from Ember last for a few days while Julia's boost from Reynard the Fox lasted for significantly longer. However, this could be due to Julia refraining from casting spells whereas Alice was actively training high-level spells.