If you like to play mage toons – Fiddlesticks, Karthus, Ryze – magic penetration can be a cheap start for your runebook. For a lot of mages, though, it’s a tough purchase to justify because of the way magic penetration works. Penetration only affects magic resistance your target currently has. It’s also the last debuff applied in a string of magic resistance debuffs. It works like this: fixed resistance is applied first, so Karthus’ wall, for instance, which gives a -35 magic resistance debuff. Then comes percentage based debuffs, like Void Staff. Then, after all that has been applied you get your magic penetration debuff.

The real kicker is that penetration can’t take your enemy’s resistance below zero. Only flat resistance can do that. If you’re in some higher tier play and you’ve got enemies stacking up MR, then penetration is a great choice. It adds a level of damage you can only get from Abyssal Staff in game. If you’re a more casual player like myself, then you might see more benefit from flat AP, mana regen, or even flat health.