Baneblade

Cooldown: 1200 - 1320 hours, varies by skill
Syntax: cast 'baneblade' <weapon>

A working with great potential, the baneblade spell draws on the natural affinity of earth magics towards forging, combining it with the ethereal malice of void to create a weapon of fell power. This spell may be cast on any weapon; the first such casting will turn the damage type to negative, and grant a small chance of afflicting one struck by the weapon with a brief and minor -- but stacking -- vulnerability to negative damage. Subsequent castings will increase this effect, prolonging the duration, extending the odds of leaving the vulnerability, and other related empowerments.

In addition, if this spell continues to be applied to the same weapon, eventually other traits will become evident, some of which are quite powerful. No one weapon may be so empowered more than thirteen times. Note that forging a baneblade takes substantial effort, resulting in a rather extended cooldown; unlike most effects, this will not expire upon the death of the caster.