Rings of magical power, as encountered in the second Amber Pentology (the Merlin Chronicles). A spikard usually appears as some sort of ring, sometimes ornate, sometimes plain, that a person can easily wear. Those with magical senses can determine that the Spikard itself is connected to many sources of magical power in Shadow?, often too many to easily count or analyze without long-term investigation. Once worn, even a lesser mage is capable of drawing upon a great amount of magical power, and can often draw upon the Spikard's power to assist them in creating even more impressive spells.
However, there are risks. Given the sheer number of Shadow power sources connecting to the Spikard, it is very hard to determine if any of them have been tainted. Also, it is a common trick to place heavy compulsions on the spikard ring itself, compelling the user to act in various ways that could be contrary to their base natures.
A classic example of a source of potent power coming with hidden strings attached.