How many kills would I need to make with Haunting guise to make it a net gain if I sell it? How realistic is it to pull it off? I think it's about 2, which characters would you get it on?

And all this for 115 gold less than a Rod...oh wait.

it's simple! (just kidding, it's not.)

follow these quick and easy steps:
1. take your target's current magic resistance (this value would be after all modifiers like buffs and magic resist reduction).
2. apply your magic penetration to the value in step 1 by first subtracting your flat magic penetration, and then removing a portion equal to your magic penetration % value. (e.g., if your target has 55 magic resist and you have 29 | 10% penetration then you would do: [ (55 - 29) * 0.9 ]
3. take the value from step 2 (we'll call it 'x' within this step) and run it through this formula: [ 100 / ( 100 + x ) ]

the result of step 3 is the damage modifier that is applied to your raw damage (like, base damages + bonus damage from ap ratios, masteries, etc) to determine how much damage your target will take. for example, if your target has 20 magic resistance after penetration is factored, then your damage modifier would be 0.8333 and therefore your target would only take about 83% of the damage you're throwing at them.

so, now to find out how effective a haunting guise would be we need more steps.
WE'VE ONLY JUST BEGUN.

4. take your champion's raw damage and modify it by the damage modifier you got from step 3. this is your 'before haunting guise' value.
5. run through steps 2 and 3 again, but this time with 20 more magic penetration.
6. take your champion's raw damage (making sure to add in an additional 25 ability power) and modify it by the damage modifier you got from step 5. this is your 'after haunting guise' value.

the value in step 6 is how damage haunting guise is adding. this number on its own doesn't mean a whole lot, so we need to compare it to something meaningful. the most practical comparisons are to either void staff (70 ability power and 40% magic penetration) or pure ability power.

void staff comparison:
7(a). run through steps 2 and 3 again, but this time with 40% more magic penetration. note that %-based penetration stacks multiplicatively, so (for example) 10% + 40% would equal 46% and not 50%.
8(a). take your champion's raw damage (making sure to add in an additional 70 ability power) and modify it by the damage modifier you got from step 7(a). this is your 'after void staff' value.
9(a). compare your 'after haunting guise' and 'after void staff' values to get the difference in damage for a particular target. btw, void staff costs 810g more than haunting guise so take that into consideration if it's relevant.

ability power comparison:
7(b). take your champion's combined ability power ratios for whatever 'combo' you want to compare. for example, if i wanted to compare damage output for veigar's standard gatling of: baleful strike + dark matter + primordial burst, i would use the value 2.8 (which i got from 0.6 + 1.0 + 1.2).
8(b). modify the ratio from step 7(b) by the value you got from step 3.
9(b). take the damage total you got from step 6, subtract the damage total you got from step 4, and then divide the difference by the ratio you got from step 8(b). this is the amount of ability power you would need to match the damage increase of a haunting guise.


~~ a wild example appears! ~~

i'm playing veigar. i'm level 18 and i have 21/0/9 masteries, flat ap quints, scaling ap seals + glyphs, magic penetration marks, two doran's rings, sorc shoes, deathcap, wota, and 100 ability power from last hitting with baleful strike. this gives me:

-a base damage of 1080 on my 'baleful strike + dark matter + primordial burst' gatling.
-a combined ap ratio of 2.8 for my gatling.
-a total of 582 ability power.
-a total of 28.5 | 10% magic penetration.

graves is on the enemy team and is my primary target since he's a tanky dps burst-mage assassin carry. he's also level 18, and hasn't built any magic resist. that means he has 30 magic resist outside of combat, but 60 magic resist while in combat thanks to his passive. i want to know how good a haunting guise would be considering that i want to deal the most damage i can to graves while his passive is up. since a void staff isn't quite justified in this case, i'll only do the comparison to pure ability power stacking.

1. 60
2. 28.35
3. ~0.78
4. [ 1080 + ( 582 * 2.8 ) ] * 0.78 = ~2113
5. ~0.91
6. [ 1080 + ( 615 * 2.8 ) ] * 0.91 = ~2550 (note: the 25 ap from guise is modified by the ap increases from deathcap and masteries)
7. 2.8
8. ~2.2
9. ~198

from our example we can see that against this graves we would need about 147 ability power to match the damage increase a haunting guise would give. note that it's only 147 ap since it would get boosted to ~198 thanks to the ap boost from deathcap and masteries. so, yeah...haunting guise can be pretty strong but it's really hard to gauge that since the numbers in your ability tooltips don't change and the math behind it isn't something that you'll dive into while in a game.

(apologies in advance for any typos/errors. it's a wall of text so i assume they exist...)

I think all the math makes sense after it being pointed out that Graves is a tanky dps burst-mage assassin carry.

The world just seems more right.

It is very good on leblanc. The key to making leblanc decent mid to late is to maximize magic pen. since her scaling is so poor and almost all of her damage is based on base damage of her skills I find maximizing magic pen makes it so people cannot ignore you late game. HG, Dcap, Voidstaff, abyssal, sorcerers shoes, X....Makes you very formidable at all stages of the game pre-50 minutes. The key is to snowball hard of her early game. If you cannot kill your mid you need to wander and pick up kills because her farming is very poor.

^The only char i buy HG on.

loled

I lol'd too because it is true.

I get it rather frequently on LeBlanc.

there's an item called haunting guise? since when?

I get it on someone such as annie was has mana refunds