Reaper's Coterie

The target weapon becomes a reservoir of spiritual energy, fueling its destructive power by drawing in a tiny fragment of the essence of each creature it is used to defeat.

When the targeted weapon is used to reduce a living creature to 0 or fewer hit points, it gains a +1 profane bonus on damage rolls.

This bonus increases by 1 for each creature reduced to 0 hit points or fewer with that weapon, up to a maximum bonus equal to 1/2 your caster level.