This appears as a finely-wrought necklace in the form of a golden spider web hanging from thin gold chain. It will radiate magic.
In testing, this will seem to allow the user to cast a Web spell 3 times per day, as if by a 10th level mage. It will have a 20 foot radius area of effect, and a range of 200 feet.
When first used in battle, the necklace will go off centered on the wearer, no matter what range or direction they desired. The wearer gets no Saving Throw, but those standing nearby, who are within the area of effect, do get one, as per the spell.
Image: Dew on a spider's web in the morning. by Luc Viatour, released under a Creative Commons Attribution-Share Alike 3.0 Unported License, modified in Inkscape by David