I'm working on the world first and worry about adapting specific game systems to it later. Though I don't think there's actually that much that would need adapting. Character races, equipment lists, and some new monsters should be covering almost everything. The only really setting unique things that come to my mind are mechanics for demonic corruption and a kind of blood magic, which I havn't really developed yet.

Magic and Spellcasters

In the world of the Ancient Lands, there is only a single supernatural force at work. Life force, magic, spirits, demons, souls; it's all the same basic energy that is found inside and between everything. This energy is what gives living creatures their strength and make them grow and heal injuries and disease. It is also what creates the souls of mortal creatures and in places where the energies of the landscape are strong, they manifest in sentient spirits of imense power.
The arts of magic are the mastery of the ability to not just call on the life energy within oneself, but to extend ones mental control to the energies around one and even inside other objects and beings. When casting a spell, a spellcaster is sending a ripple through the energies surrounding him to manipulate the energies within creatures and objects to their desire, just as normal people control the energies within their arms and legs. Magic spells can make plants grow or move, split or shove away rocks, create lightning and set things on fire, and even take control of other creatures thoughts. To be able to manipulate the life force and magical energies at a distance, a spellcaster needs to build up a reserve of energy separate from their own life force, which they accomplish by many years of training and meditation. Once this reserve is spend, they can no longer cast any spells until the energies are restored. People who have mastered magic, and creatures that are naturally magical, require almost no effort to regain their spend magical energy. Their energy reserve has become a natural part of their own being and during rest energies from the surrounding environment automatically move in to fill the void and restore the natural balance of energies, like water after a bucket has been filled from a lake. Compared to the amounts of energy in the natural world, even the most powerful spellcasters can store only such a small amount that there is no noticable drain of life force in the creatures and plants arround them.

Blood Magic
In the earliest days of the humanid peoples the magical abilities of spirits and other magical creatures was not yet entirely understood and the earliest spellcasters did not yet have the ability to create reserves of magical energies to power their spells. But they realized that magical energy and life force is essentially the same and in no other part of a mortal creature is its life force as strong and concentrated as in the blood. Not only could these early spellcasters use the energy in their own blood to cast their spells, they could also use the life force of other living creatures.
When the more common forms of magic, or spirit magic, were discovered, this blood magic was soon abandoned. The storing of magical energies from the environment is not only much more effective, but also much less painful and gruesome. However, unlike many shamans and wizards would like to believe, the traditions of blood mages never entirely disappeared and continued to exist in some small remote places for all the thousands of years. Pure, traditional blood mages are almost unheard of in the present age. The advantages of using the magical energies of the surrounding natural world are just too great for any spellcasters to ignore. But there are still some advantages that blood magic has over spirit magic, which allowed the ancient traditions to survive. By drawing the life force from their own blood, blood mages can significally extend their reserves of magical energy and even make their spells stronger and more powerful than they would usually be able to. And by using the life force of other living creatures, blood mages can have access to vast amounts of magical energy for prolonged rituals and large scale magical effects, that would take ordinary spellcasters days or weeks and the use of rare and highly expensive substances to store for later use. Because of the violent and gruesome nature of blood magic, most people regard it as savage and horrific, but contrary to common believes, blood magic itself does not have a corrupting effect on those who use it or are affected by it.

Demonic Energy and Corruption
Outside the natural world that consists of the material world and the Spiritworld exists a realm of infinite time and space that is simply called the Void. Just as there are spirits in the natural energies of the spiritworld there are also beings born from concentrations of the energies of the Void, called demons. When spellcasters learns a way to create a connection to the Void, it allows them to draw some of its energies to their own world and use it as an additional source of magical power in addition to their own. With these energies they can create spells that can not be cast with magical energies of the natural world and it also greatly increases the number of spells they can cast before their reserves are exhausted.
However, the natural world and the Void are realms of completely different laws that are conflicting with each other and bringing energies from the Void into the natural world causes it to become warped and twisted, an effect known as corruption. The demonic energies are toxic to any living things that are affected by it. Spellcasters who have mastered their use can learn to resist its effects and may show only very little signs of the massive exposure they recieve from many years of casting demonic spells. However the land around their homes and lairs becomes significantly affected, making these places hostile to all living things, turning them weak and sick and eventually killing them.
But corruption is not simply a drain of life force, but a warping and twisting of it, and corrupted life force is still able to support life, at least in a manner of speaking. When creatures becomes entirely corrupted but do not simply fall dead, they turn into the undead. Zombies and skeletons are corpses that have become powered by corrupted energy after their death, while those who have never really died become ghouls or wights. In rare cases, a person dies but the corrupted energies that have been part of him continues to exist as a faint afterimage of a creature known as a shadow. When the corruption has been so complete that the soul itself has become corrupted, it continues to exist even after the body has died as a wraith.