Believable magic, like ordinary physicis, operates according to some invariable laws that always result in some kind of cost or 'bounce back'. The grater the magic, the more it should cost the character physically, emotionally, and spiritually.
“ A revolt or attack happens against a strong city/kingdom. All the losers of the battle that can be captured are brutally crucified along the roads that enter the kingdom to show their strength and as a demonstration of what happens to their enemies.”
“ After killing certain animals, if you drink their blood while performing a sort of ritual you will have visions and/or gain specific powers for an amount of time.”
“ Basq doesn't exist. He occupies a point in space--a single X,Y,Z point in the Cartesian plane, but he's not made of anything. People looking at him see whatever they want to see. Or what he wants you to see. Usually a mix. Just the same, he cannot be hurt swords or arrows. Only things that deal damage to an area (fireballs, gases) can damage him. Or weapons that can cut through an infinitely small point.”
