“ 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.”
“ The massive blade known as Consequences carries several potent enchantments of battle, but also has a frustrating quirk: Its wielder finds himself unable to put it down until he enters the presence of a magistrate or other authority. Even then, it instantly returns to his hands if he has committed murder and fails to confess. Unless he somehow resists the blade's magic, the weilder's hands then run with fresh blood; the judgmental blade fights his every motion until he confesses his crimes.”