The Soul Blade

This blade requires sacrifice to reach full power.
Once 666 souls have been slain, this weapon
captures the soul of the next medium (or larger)
creature it defeats.
This sword may release the soul to inflict an
extra 2d8 nectrotic damage onto any foe it hits.
If this blade does not kill at least once every 3 days,
you must succeed on a DC13 Const save or die.
You have a +1 bonus to attack and damage rolls.