Mana Steal is a frequently encountered trait on items (generally weapons). It allows the user to replenish their Mana with damage dealt against the enemy. The steal comes from how much damage you do to the monster. So 5% Mana Stolen will return 5% of the damage you actually deal, rather than being based on the damage your weapon deals. Life and Mana steal only work on Melee, and physical ranged attacks. There are several magical suffixes that enable Mana Steal, so the effect may occasionally be found on Magic and Rare Items.