“ 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.”
“ On a location with numerous webs, and at least one big spider, there is a something inside a cocoon. It is humanoid in shape, still moving. If the heroes free it (not before they kill or drive away the spiders), they meet a ... zombie!
The poor zombie wandered the dungeon alone, and tried to kill a big living creature (= a spider). The spider used the usual treatment, even if this victim did not look tasty. The zombie can be easily killed as any other zombie. It got but several doses of spider-poison, so can be something worth if it is extracted. You can mention to a druid or ranger the fact the spider had no poison anymore.”
“ It has been said that within the mountains (or was it the darklands?) there dwells a fluffy creature of immense cuteness. This creature is so adorable that all who encounter it are filled with joy so great their mortal forms cannot contain it. And they explode. Several magic institutions are desperate to study the creature or better yet capture one to prove the theory of an elemental plane of 'D'Awwww'.”