Invocation is a branch of magic where the practicioners attune themselves to one of the elemental or mental planes and then taps into the power of that plane to produce magic. Alternatively, power may be granted by a powerful being, such as a primal spirit, an elder dragon, or a collective of spirits. Invokers tend to be more limited than wizards, but yet retain much versatility, as they can also have power of concepts related to their sphere of influence.
For example, one attuned to an elemental plane of fire will be able to use powers aligned with concepts related to fire, such as strength, passion, and renewal. As well, peripheral elements can be accessed. One attuned to the elemental plane of wind, for example could use powers that fall directly under the elementals of water, lightning, and frost, so long as they still fit the concept of wind.
Attunement to a mental plane tends to grant a broader range of powers due to having a wider sphere of influence. Those attuned to a mental plane can produce sacred (or profane) Nitor that heals those who are aligned with the mental plane and harms those who are opposed to it. Those who are neutral can be harmed, healed, or receive no effect, based upon the invoker's choice.
Some invokers, rather than being attuned to a plane of existence, instead receive power directly from one of the draconic lords, in exchange for service.