Spells leave a mark on the objects they enchant and the people they inhabit. With certain spells, mages can even see the magical auras of spells as they affect both people and objects. Vincal didn’t simply want to see magic, she wanted to target it. Thus, she enchanted her halberd with a unique ability: honing in on magical auras.
Slayer of the Wise is a magical halberd, having the base stats of a halberd in whatever system you are using. Its unique effect increases the attack roll made against opponents that have spells, spell-like abilities, or are using magical items (armor, weapons, items, etc) equal to the ½ the highest value the magical item grants (rounded down, minimum +1).
For example, if a player using Slayer of the Wise attacks a mage that has access to 4th level spells, she gains a +2 to attacking that mage. If she attacks an opponent with a +2 flaming sword, she gains +1 when attacking that enemy. If she attacks an enemy buffed by a spell, she gets a bonus against them equal to ½ the highest level of the spell(s) affecting that opponent.
No matter how many magical items, bonuses, or spells an opponent has, the halberd’s bonus to attack only applies to the highest value. For example: If she attacks a mage that has access to 7th level spells and has a +5 amulet of deflection, she gets a +3 to attacking the mage.
Slayer of the Wise deals additional damage equal to half the spell’s level on anything that is pure magic. For example, summoned creatures made of energy and magical walls. If there is a shield completely created by magic, she gets the bonus damage when she hits the shield, not the person using it.
The user does not gain any special awareness of magic or if the target she hits is magical. The effect is passive and not under the player’s control to apply or cancel it. Slayer of the Wise also offers no way of knowing where the more powerful of two magical auras is located.
Slayer of the Wise is more powerful in campaigns where the enemies use lots of magic items or cast spells. If that is the case for your game, you may want to give it additional qualities. In any game, you could reduce the bonus to attack by 1 if there is a stronger magical source within sixty feet of the halberd (since it seeks the strongest magic).
If you’re playing a d20 game (Pathfinder, D&D 3.5 etc.), you can make this ability a +1 or +2 bonus. Do what fits your game the best.
Whatever makes it best for your game, change it. The base item works, but this is about making something great for your campaign. If it needs more or less flair to fit or balance, always do that.