Entropic,Greater (3.5e Equipment)
From D&D Wiki
As Entropic, except in addition to the normal effects of a critical hit the target is also struck by a negative energy burst as the spell cast by a level 15 spellcaster. The negative energy burst only occurs if a natural 20 is made on the critical roll. Those within the area of effect are also subject to damage by the burst. This property cannot be applied to ammunition.
(Major Necromancy) Caster level 20, Caster must be evil, Craft Magic Arms and Armor , Greater Harm , Energy drain , Negative Energy Burst, Entropic, Price +4 bonus, Must be crafted in a major negative energy-dominant area.