Lore24 - The Sovereignty - Magic of the Land

Eldritch power flows through the very lands of Aedes. In places where nature is strong, wizards can harness magical energy to cast mighty spells and rituals. According to these arcanists, the force of magic is the very lifeblood of Aedes, causing plants to grow, animals to procreate, even humanity to give birth to new generations. The force of magic is the force of creation itself.

Heresy, says the Ecclesiarchical Church of Phaeton. There is only one source of life, and that is the light of Phaeton, the Sun's holy radiance. According to the Church, humankind is destined to tame the land, not live in awe of it. To slay the monsters spawned by arcane forces, not try to understand them. As such, wizardry is regarded with suspicion in Aurora and the Hartlands. 

The origin of this suspicion may lie in the very first chapters of the Holy Codex. According to the Codex, Phaeton shaped the world of Aedes out of the corpse of the Mother of Monsters. According to Church theologians, the magic of the land is the emanation of the remains of the Mother of Monsters, while Phaeton's light seeks to tame it.

Whatever myth or legend tells us, the effects of magic are real, as are the influences of human civilization on the land's power. The further one is from civilization, the stronger the spells a wizard is able to cast. Conversely, a mage's power is weakened the larger the human population is around him. Is this perhaps the effect of the ancient struggle between Phaeton and the Mother of Monsters? Only the very wise deign to know the answer.

Notably, only human and dwarven civilization impacts the power that flows through the land. The elves of Aedes, having a greater understanding of the land and magic, know how to live and build in harmony with nature. Their cities leave the flow of magic unimpeded.

OSR

In old school game rules, this impacts the maximum level of spells arcane magic-users are able to cast. 

Plains/Desert/Hills:                      level 3
Forest/Mountain/Water:               level 4
Swamp/Marsh:                             level 6
Heavy Forest/High Mountains:    level 8

Light Cultivation:    -2
Heavy Cultivation:  -4
Large City:              -6

Cortex

For Cortex Prime, the power of the land manifests as a die added to the wizard's dice pool, as follows:

Plains/Desert/Hills:                      d6
Forest/Mountain/Water:               d8
Swamp/Marsh:                             d10
Heavy Forest/High Mountains:    d12

Light Cultivation:    1 step down
Heavy Cultivation:  2 steps down
Large City:              3 steps down

Eldritch Locations

Now, there are places on Aedes where arcane power flows more potently even than in ancient glades or between stark mountain crags. In these eldritch locations, arcane power is one level higher than in even the most mysterious forests and mountains (level 9 in OSR games, d12 for Cortex Prime). These locations are veritable wellsprings of magic and life, and are very rare and often tied to ancient secrets tied to either Phaeton or the Mother of Monsters.