Talk:Xerath/Strategy/@comment-190.177.232.29-20111204143212/@comment-64.180.243.135-20111215231124

"flat Magic pen screws with his magic pen %" (Caution: needlessly long gripe ahead)

I kinda boggle at this and the warnings against stacking mpen in the article. Sure, having flat mpen means you get a smaller number out of Locus... but your net mpen is ALWAYS significantly higher with both. For raw damage on a nuke champ, more is almost always better imo.

Unless your planned build will be hurting for CDR, I'd usually recommend going with Sorc's Shoes. Early game Xerath eats mana, Locus' cooldown is huge and the small mpen % combined with small damage numbers and usually small amounts of enemy MR means flat mpen is gonna be much more effective and reliable during the laning phase (if you have mpen marks, rank 1 Locus is worth a whopping 2 MR against a 30 MR target; 3 MR without mpen runes!). Plus, even in the late game it's not always worth the time or risk to set up Locus, so it's nice to have some other mpen sources to fall back on.

Just to show that an mpen item is still worth it, let's assume that you're level 18 with 600 AP, 8.55 flat mpen from runes, 10% mpen from Arcane Knowledge, with Locus set up for another 30% mpen, and casting Arcanopulse for 235+(600*0.6) = 595 base magic damage.
 * WARNING: excessive maths ahead***

1. You're staring down a squishy who never bought a single MR item for some reason and has no MR runes, thus sitting at a measly 30 MR. Without any mpen items: 30 - 8.55 = 21.45 21.45 * 0.9 * 0.7 ~= 13.5 13.5 MR ~= 12% reduction Your Arcanopulse blasts them for an impressive 595 * 0.88 ~= 524 damage.

Now let's say you had Sorc's Shoes: 30 - 28.55 = 1.45 1.45 * 0.9 * 0.7 ~= 1 (oh man, %mpen is reduced to almost nothing) 1 MR = 1% reduction Your Arcanopulse fries them for 595 * 0.99 = 589, nearly full damage. Only 65 more damage--so what, right? Except that it's worth the equivalent of 125 extra AP compared to the no-mpen-items example (235+(725*0.6) = 670; 670 * 0.88 ~= 590).

2. You've got a melee DPS coming your way. They didn't buy any MR items, but they have MR/lvl runes and natural growth for a total of 76.8 MR. Without any mpen items: 76.8 - 8.55 = 68.25 68.25 * 0.9 * 0.7 ~= 43 43 MR ~= 30% reduction Your Arcanopulse takes a chunk out of them, dealing 595 * 0.7 ~= 417 damage.

Now with Sorc's Shoes again: 76.8 - 28.55 = 48.25 48.25 * 0.9 * 0.7 ~= 30 (our %mpen real reduction dropped from 25 to 18, boo) 30 MR ~= 23% reduction Your Arcanopulse blasts them for 595 * 0.77 ~= 458 damage. OK, only 41 more damage this time, pretty lame. Still worth the equivalent of 100 AP (235+(700*0.6) = 655; 655 * 0.7 ~= 459).

3. Let's say you went with Ionian boots. Just so happens that the game has dragged long, and your team was unusually Magic-heavy. The enemy team wised up, and now they all have at least 100 MR. Without any mpen items: 100 - 8.55 = 91.45 91.45 * 0.9 * 0.7 ~= 58 58 MR ~= 37% reduction Locus is performing admirably, cutting off 33 MR when combined with Arcane Knowledge. Arcanopulse zaps the least MR-stacked of your MR-stacked opponents for 375 damage.

Now let's say you switch out your last item with a Void Staff. Multiplicative stacking screws it over, right?: 100 - 8.55 = 91.45 91.45 * 0.9 * 0.7 * 0.6 ~= 35 35 MR ~= 26% reduction You might think it gets screwed over, but it's still better than flat mpen at this point, slashing an additional 23 MR for a total of 56 MR ignored thanks to stacked %mpen. Your Arcanopulse hits them for a respectable 440 damage. Once again, that's 65 more damage, equivalent to an additional 125 AP. OK, maybe slightly less since you'd probably have 15 more AP or so if you took something bigger than Void Staff for a final AP item, but still. On the other hand, Abyssal Scepter might be more useful if you need the MR, but it's worth keeping in mind that Arcanopulse and ult reach beyond the aura with Locus going, so Void Staff might be a little more reliable for picking off a fleeing enemy, etc.

major tl;dr - don't worry about "screwing over" Locus, damage-wise it's still well worth fitting in at least 1 mpen item if you have room in your build.