the Markey are the descendants of humans from Terra who have been transported to Halkeginia and formed its Noble class of Mages. Relying on Elemental Magic. Unlike average humans from history of Code Geass, the Markey were actually the remnants of the Geass race (as glimpsed when Lelouch accepts the contract in Code Geass) who escaped the destruction of the formation of the first two Codes. Markey magic is reliant on the brain because it is the projection of the personal will on the world, as is the case for all other magics which the immortals have experienced. In opposition to this is Void magic, which works by messing with an object's sense of Erebos, creating effects similar to Geass, but also capable of drilling between realities in the same way Geass Gates do, as opposed to the way other magics do.
Geass race on Terra is widely gifted with magic (source of tales of magic lasting into modern eras)
Geass race attempts to overcome mortality through experimentation, believes to have found solution in the production of the Code
Geass race mass extinction forms the primary Codes of Terra at the cost of countless lives
Remnants of Geass race use magic to escape Terra to Halkeginia
Become the Markey race, forgetting about their history and source
Markey race declines in magical power exponentially after forgetting the source
come to Brimir's time, he invents (or discovers through Sorcerous Origins?) Void Magic, the selfsame magic which leads to the development of the Code
brimir uses void magic to fend off enemy races the Elves and the Varyag
brimir founds faith centred around worshipping him, realizing the power of the collective unconscious believing in him lends him further power because void magic is connected directly to the collective conscious via a stint directly (and dangerously) through Erebos
brimir attempts suicide ritual to genocide elves but is stopped by Sasha, his elven familiar/lover killing him with Derflinger
Derflinger is a pseudo-Code Bearer, not being a fully sentient creature, but a sentience nonetheless. capable of absorbing magic. through absorbing/recombining the souls of the inheritors of brimir's void magic, derflinger has the capability to be transformed into a true code, combining with the last soul (Louise).
markey go through another age of exponential power decay, but through brimir's previous efforts they have successfully been installed as the Noble class, and the Vargay indigenous to Halkeginia have been resigned to being the Commoner class
story of EIT follows after this