Soul-Essence Dagger (3.5e Equipment)
From D&D Wiki
Soul-Essence Dagger: This black and twisted +1 dagger has serrated edges on each side. Each time a creature of intelligence 6 or greater is killed by this dagger, a charge is added to the dagger for each Hit Die of the creature killed. The dagger holds 100 charges at its maximum, but is purchased or found with 0 charges.
The following abilities are available (with caster level of 7 or the caster level of the wielder, whichever is higher and DC 10 + spell level + wielder's Int modifier):