Original Post — Direct link

Hello, I spent hours in testing and calculating in practice tool, and I can say for sure that Shadowflame is NOT giving the Magic pen it is supposed to give.

The description in game says "max benefit when the target has 1000 or less health"

The description from wiki says basically the same but more detailed :
Formula: 10 + 1 for every -150 target's current health starting from 2500 target's current health and capped at 1000 target's current health.
Source : https://leagueoflegends.fandom.com/wiki/Shadowflame?so=search (hover the passive description on the right of the page to get the formula)

Unless it is a bug specific to the practice tool, which I highly doubt, Shadowflame is granting 1.6 to 2.9 less magic pen than intended/stated in the tooltip. It's not a gamebreaking bug, but nobody seem to be aware of that and this bug persists at least since Shadowflame went live (probably was already on PBE but I did not test there)

I tried on different champions and I never saw Shadowflame working as intended. The easiest way to test is to remove any damaging rune that could interfere, then start a practice tool.
Buy only shadowflame, put a target dummy with minimum health (1000) and 20 MR.
Attack the target dummy with a simple spell that does a single instance of damage like annie Q.
Note the result, then sell Shadowflame, and buy instead sorcerer shoes + needlessly large wand + blasting wand to have the same AP than shadowflame.
Attack again the target dummy with that new build using the same spell like above and note the result.
In both situations you have 100 AP so your base damage in the same, only Mpen changes with the 2 builds. Since target has 1000 hp or less, shadowflame is supposed to grant 20 Mpen against that target, and pre mitigation damage should be equal to post mitigation damage (you have 20 mpen and target has 20MR, but the values are always lower than the predictions because in that case, for some reason in that situation Shadowflame gives 17.1 Mpen instead of 20.
Additonally, the sorcerer shoes 100 ap build always do slightly more damage than Shadowflame only.
But you are supposed to benefit from 2 additional magic pen against 1000 HP targets when you shadowflame instead of sorc shoes.
After making the math, I could find out that Shadowflame grants 17.1 Magic Pen exactly on targets with 1000 current health or less. I tried to reduce target below 1000 current health to try but it always give the same results than 1000 hp target, so it does not scale below 1000 current health as stated in the tooltip. Also it grants 8.4 magic pen instead of the intended 10 on targets with 2500 current health or above, and it doesn't decrease further as the health of target increase.

The general formula to calculate post mitigation damage is :
Post mitigation damage = base damage*100/(100+armor)
It is the same formula for magic resist and ability haste
Source : https://leagueoflegends.fandom.com/wiki/Armor?so=search
Graph for ability haste into cdr or armor/mr into damage reduction : https://www.desmos.com/calculator/atkt8mr6qf?lang=fr

So what I did next is picking Annie because her Q is simple for tests, I removed any rune that give any offensive benefit to not interfere with the test, I went for Shadowflame/Rabaddon/Lost chapter/Spellthief edge, which granted me exactly 374 AP so my Q does exactly 500 base damage at rank 5.

Then I put a target dummy with 20 mr and I tested my Q damage on it for each HP value possible.
I tested for 888, 1000, 1100, 1200, 1300, 1400, 1500, 1600, 1700, 1800, 1900, 2000, 2100, 2200, 2300, 2400, 2500, 2600 hp
(For the 888, it's 2 auto before Qing at Annie level 9, was just to make sure the magic pen doesn't increase further if target goes below 1000 current health)

Here you can see all my data in a google sheet screenshot : https://ibb.co/8b22D3Q
You can easily reproduce my results step by step with all the infos I gave.
For the Expected Mpen column, I am just following the wiki chart (but from my testing, the formula is linear and not working per 150 health threshold as stated in the wiki - but it's really a detail, whatever.
For the Calculated Mpen, the formula is : MR-(-100*Post mitigation damage+100*500)/Post mitigation damage
For the Expected dom, the formula is just 500*100/(100+MR - Expected Mpen)
The Damage done is just the post mitigation damage displayed on my screen at the moment my spell hit

The column about the damage done with sorcerer shoes instead of Shadowflame is meant to confirm there is no error in formula or math, in that column every observed result match with the expected result. (Specifically, 500 base damage on a 20 mr target when you have 18 magic pen turns into 490.2 post mitigation damage which is what the formula predicted. 500*100/(100+2) = 490.19

This is by the way what should happen with Shadowflame on the 1300 and 1400 current health target but the damage done is 478 and 475 instead of 490.19 because the Mpen granted by Shadowflame is 15.4 and 14.7 instead of 18 and 18.

Additionally, I did not test against shield but I assume I would get the same values than I get while attacking a 1000 hp target or below.
Also for those who wondered, it works as usual Magic pen for the void staff interaction (and similar effects like morde E passive), the % magic pen is applied BEFORE the flat magic pen, just like sorcerer shoes.

TLDR : Unless it is a bug specific to the practice tool, which I highly doubt, Shadowflame is granting 1.6 to 2.9 less magic pen than intended/stated in the tooltip, depending on enemy current health

It's obviously not intended. And it's lasting for very long already. Any thoughts? I'm going to contact support to ask and see what they answer. If you think I am missing something, feel free to correct, but I really triple checked all the informations I gave there!

External link →
20 days ago - /u/PhreakRiot - Direct link

Originally posted by RobbinDeBank

It’s literal garbage against shield. It’s not a bad item, but it’s a lie that shadowflame is anti-shield.

Yeah that's pretty much how I view it as well. It's a flat pen item made to kill squishies. Getting 10 more flat pen against a 4 item Nautilus just because he pressed W is not special.