The magic they can utilize is a type of life magic because it’s based on life energy. A skilled user of this type of magic can pull the life force out of nearby living organisms to fuel their powers. While in the book Toby and his father only use this power for offensive magic (they create burning golden energy and walls of force), the magic can be used for so much more to include: healing, moving objects with a power similar to telekinesis, warping time and space to teleport, and even ripping open gateways to other dimensions. Yet, there is always a price for such actions, for nothing is free.
To use this type of magic, you have to have an ample supply of life energy. As seen in the book, Toby first pulls life from the plants around him when he’s in the forest. While he’s channeling energy, the flora around him wilts and dies. This connection to the life around him gives him a radar of sorts, and allows him to “feel” the things surrounding him. Users of this type of magic can also pull life from other beings other than plants. The danger of this, is that the power becomes addictive and changes the magician. They can become drunk off the energy and will always want more and more, and many undisciplined users of this life magic have succumbed to the desire and have had to be eliminated (yes there are organizations dedicated to keeping magic users in check).
There have been other noted side-effects from overdrawing on one’s own energy reserve to include loss of memory, blackouts, migraines, hallucinations, hearing voices, and even bad indigestion. To get around this, users of life magic have developed ways of storing reserves. Some will store energy in trinkets for later use, while others will hold energy directly in their own bodies. Storing energy in their own bodies will only work for a temporary time, as it will dissipate within a day.
The use of magic of any sorts is dangerous in the world of the Dark Tyrant series, for harnessing such powers generally draws the attention of unwanted beings. In the case of Toby, he drew the attention of a powerful demon in the Utah wilderness. While magic can be a boon, there are always consequences.