Bat is a suffix that can appear on Magic and Rare Items. It provides a small Mana Steal bonus. It can start spawning on items at Item Level 7, but will not be able to spawn on all items until level 35. Some items also are not able to acquire full effect. The steal comes from how much damage you do to the monster. So 5% Mana Stolen will return 5% of the damage you do to a monster. Mana steal only works on melee and physical ranged attacks.