This is just a short explanation of just what MP Eater is in Maplestory. This is a skill for the jobs of Wizard (either Fire/Poison or Ice/Lightning) or Cleric. So,any mage can have this skill after the first job advancement. Essentially, the function of MP Eater is to help keep your magic points higher, so you cancast more spells before you need to drink potions or rest, however you like to get your MP back. It is a passive skill, so once you have put points into it, you do not need to activate it or set it to a hotkey.
How does MP(MapleStory Mesos) Eater work?
Each time you attack, there is a chance that you drain some MP from the enemy. The amount you drain is dependent upon a combination of the Maximum MP of the enemy and the number of skill points you have allocated to it. The formula for its chance of activations is: 10% + 1% per skill point. So, if you have 7 skill points in MP Eater, there is a (10% + 7% = ) 17%, chance that MP Eater activates whenever you make a magic attack on something. Since the master level(Maplestory powerleveling) of MP Eater is 20 points, there is a cap of a (10% + 20% = ) 30% chance that MP Eater activates for each attack.