|A weapon said to have once been used to keep unruly imprisoned wizards in line; it violently disrupts their ability to use magic, inflicting wild and unpredictable residual magical effects on those it strikes.|
+5 demon whip
Predictably enough, "Spellbinder" is useful when facing spellcasters, but somewhat unimpressive against more mundane opponents. A demon whip is a powerful weapon, but you can certainly find or enchant better, especially when you consider that its antimagic brand inflicts no additional damage and penalizes your own spellcasting abilities.
That being said, when used against spellcasters, it handicaps their ability to use magic against you and afflicts them with an unpredictable array of miscast effects. This more than makes up for the Spellbinder's middling offensive capabilities, but can occasionally be hazardous to your health; depending on what you're attacking, some miscast effects can create dangerous clouds, elemental explosions, or summon dangerous neutral monsters.
The Spellbinder was added in 0.14.
Prior to 0.15, "Spellbinder"'s enchantment level was +3, +6.