In the story I am writing the universe itself was create from a magical storm in a realm known to mortals as The Beyond.
Now it is believed that there is the Realm of Reality in which the story takes place, and The Beyond a plane of existence that shadows the mortal plane. While The Beyond and the Mortal plane cannot see each other or physically interact with each other. The presences of The Beyond can be felt in the air by certain individuals, your mages per say. This energy is referred to as Aether. Aether can be used as both a creative and destructive force.
Your magic users are capable of pulling Aether from the beyond to cast magic. However, in the rules I am setting out, the immaterial of The Beyond cannot stay in the material world for long. This leads to casting magic being mentally draining. The stronger the mind the greater the feat can be achieved.
Also drawing too much power from the Beyond creates a ripple in The Beyond drawing spirits and wraiths of the immaterial plane to try enter the realm of reality and kill whoever is on the other side. This is as the entities of the Beyond view the mortal plain with hate and disgust, seeing Reality as a mistake that should never have happened.
Lastly I have an idea that to keep something made of the immaterial in the realm of reality one needs to use blood magic. The bigger the object the more needs to be sacrificed.
Is this enough of a cost to make the use of magic seem like a believable cost, something that someone needs to value and weigh what is worth?
Therefore, evil would seek to use blood magic to achieve power and make their achievements permanent while those who use magic for good use it for a temporary aid rather than as long term solution.