(Adapted heavily from Grim World)
A good cursed item is just as tempting for the player as it is to the character. The best cursed items are very tempting. An item that’s just bad all around isn’t nearly as interesting as an item that holds great power but comes with a cost. Consider perhaps the most infamous cursed item of them all: Tolkien's The One Ring. As a GM, don’t feel you need to always reveal every detail of the item. Sometimes it’s best to tell the player what they’re risking, but often it’s more fun to find out the item’s curse through play and then making the GM move Reveal An Unwelcome Truth.
Give the cursed item normal equipment tags and then give it a cursed item tag. Here is a list of some possible cursed item tags, but don’t be afraid to create your own. In addition, the Magic Item tag vulnerable makes for a good curse.
When this item is in your possession, something about you (such as your personality or alignment) will be completely opposite. The bold will become meek; the good will turn evil.
Periodically, the item needs to consume or soak up a specific substance (such as sunlight, blood, tree sap, etc.). If this is not done, the item will not function or begin to degrade.
When you attack with this weapon and fail, the damage is dealt to you instead.
When this item is in your possession, everyone around you will feel an inexplicable sense of disgust towards you.
An item with the Bound tag means it’s somehow attached to the wielder. It could be embedded in your skin, or cause you great pain when you put it down. Perhaps every time you attempt to rid yourself of it, somehow the world conspires to give it back. The item could bind itself to you as soon as you pick it up, or need to be used or triggered first.
When this item is in your possession, those you interact with will be deeply suspicious and think you’re up to something.
Occasionally, the item will put a troublesome impulse in your mind without you realizing it. The item might want you to steal a valuable, betray an ally, or start a fire, depending on its personality.
The item will encourage a specific emotion, such as anger, fear, sadness, or apathy.
Whether it’s magically powerful or seemingly mundane, a sentient item is one with a mind, will, and goals of its own. Maybe it’s a spirit living inside the item, or the personality of a past bearer has imprinted itself. It’s not always inherently evil, but beware what happens when you disagree.
Something (usually bad) happens when a particular trigger is hit. The trigger could be a fictional event (when the item is in moonlight), or something mechanical (like rolling a specific number).
Unlucky items will cause their bearers misfortune. Depending on the severity, this might manifest as a -1 to all rolls, or as serious complications that unexpectedly happen when the character acts. Particularly unlucky items will tend to “attach” themselves to their owner somehow, making them very difficult to get rid of (see: Bound). Often, another person must be tricked into accepting the item.