League of Legends (LoL) Question: Magic Pen and Void Staff Question

It ignores some of their magic resist. Magic resistance reduces magical damage by a percentage. Buy buying magic penetration, you yourself are ignoring a portion of their magic resistance. Say they had 100 MR and you had 30% MPen. You would be dealing damage to them as if they had 70 MR as opposed to 100.

There's flat magic penetration and percentage magic penetration. Flat simply removes the stated amount of armour or MR. So if you had 20 flat MPen, then you will simply ignore 20 MR when dealing damage. Haunting Guise, Liandry's Torment and Sorcerer's Shoes provide flat MPen.

There's also MR shred, think of how Black Cleaver works. While this also reduces a portion of their MR, this applies an MR debuff for a period of time which benefits everyone that attacks the target affected by it. The only items that provide an MR shred are Abyssal Mask and Wit's End, both of which are flat. A multitude of champion abilities though, such as Trundle's Subjugate (percentage) and Fiddlesticks' A Harmless Scarecrow (flat).

It is impossible to have the opponent be on negative MR with only percentage MR shred due to the nature of percentages and the fact that there are not enough items in League to allow for 100+% MPen. It is also very difficult to have the opponent on negative MR with flat penetration as well.

But do know that it is possible, if you can somehow manage to do it, to get the opponent on negative MR. Where as MR normally reduces magical damage by a percentage, having negative MR will increase your damage by a percentage. 100 MR will reduce damage taken by 50%. -100 MR will increase damage taken by 50%.

If you have both flat and percentage penetration, then percentage penetration is taken into account first, then flat MPen.

Void Staff provides percentage MPen, so it would work like how I explained in the first paragraph.