Blade of Eternal Darkness

The Blade of Eternal Darkness has no specific class restriction, but it is aimed more towards spellcasting classes. If a character wields this dagger and casts a damaging spell on an enemy, there is a chance that the dagger will deal 100 Shadow damage to the target as well as restoring 100 mana to the wielder.

Source
This item drops from Princess Theradras, final boss of Maraudon.