https://youtu.be/s7vhTVAbV5w
http://leagueoflegends.wikia.com/wiki/List_of_champions%27_magic_resistance
Every champ starts with either 30MR and gains zero per level, or 32.1 and gains 1.25/lvl, which puts them at 53.4 at level 18. So vs Jinx, who gains and builds no MR and typically Berserker's Boots, yes, you would do true damage if you took
+
.
Vs a tank with 100 bonus MR, 15 flat MPen simply doesn't do much. Flat MPen is best when your opponents have low MR to begin with. But AP items are always going to give a higher percentage damage increase earlier. This is why items with both AP and Pen/Red are so good.
Using CPT Pants' example, lets compare Liandry's, Zhonyas, Void Staff, and Abyssal Scepter for a first AP item. A this point I need to point out that the DoT from Liandry's ticks in half-second increments. So the correct formula will show .01*6 to represent this. Damage figures are rounded top the nearest whole number.
: (120+120)x.6=144. 144x100/(100+30-0)=111.
: (120+50)x.6=102. 1070x.01x6=64.2. 102+64.2=166.2. 166.2x100/(100+30-15)=145.
: (120+70)x.6=114. 114x100/(100+30x(1-.35))=95.
: (120+70)x.6=114. 114x100/(100+30-20)=104.
: (120+156)x.6=166. 166x100/(100+30-0)=128.
So much for flat MPen/Red early. AP reigns. But what happens when you are going vs a lvl 18 tank with 3000 raw HP and 100 bonus MR?
: 144x100/(100+53.4+100-0)=57.
: 3000x.01x6=180. 180+102=282. 282x100/(100+53.4+100-15)=118.
: 114x100/(100+(53.4+100)x(1-.35))=57
: 114x100/(100+53.4+100-20)=49.
: 166x100/(100+53.4+100)=66.
Liandry's health-based bonus damage is huge, roughly doubling your damage despite having only 15 flat MPen. Vs lower HP non-tanky champs it also has a nice MPen number and some AP so that it is effective there as well. The numbers I used here do NOT consider the effects of CC on Liandry's DoT burn damage.
Also, the order of precedence on Magic Pen and Reduction is Flat Reductions (Abyssal), %Red (?), %Pen (Void Staff), and finally Flat Pen (boots/Liandry's). So the combination of Void Staff, Liandry's, and Sorc Boots is very effective for getting the most magic damage possible.
(120+50+70)x.6=144. (144+180)x100/(100+153.4x.65-15)=175.4. That's about a 50% increase in applied damage just by buying Void Staff. No boots. If you add in the 15MPen from Sorc Boots it goes up to 191 damage.
Liandry's DoT effect has a couple down-sides. It does not stack. It only refreshes. So bursting someone will only refresh the DoT with each spell, negating a great deal of its bonus damage. So CPT Pants is right to say it is best used by pokey champs as frequently as once every 3 seconds, like
. I would never build Liandry's on an assassin. It would probably work well on Skarner, too because of its synergy with Cinderhulk. But he benefits more from AD. (See
on PBE.)
It also does damage based on current HP, rather than total HP. So again, its best when used as poke to get that tank's HP down first, and then let your carry finish them off.
On the plus side, the double damage procs from AoE spell damage. So any long range ability spamming champ can really put the hurt on an HP stacking champ. With the advent of Cinderhulk, I would not be surprised to start seeing more of Liandry's and AP poke champs like Karthus and Soraka.
Couple interesting things I've read about Liandry's: Rylai's procs it's doubled damage. Spell Vamp has full effect with Liandry's bonus damage. Unfortunately, 20% (
) of 4% is exactly .8%. So in the above case of doing 180/360 damage to Jinx, it would heal for 36/72 HP over 3 seconds in half-second ticks. Not exactly exciting.
So, Liandry's is actually pretty decent. But the meta to date has not favored the champs that benefit the most from it. And those champs are going for things like RoA + Seraph's Embrace because they have good and useful build-paths, have good stats when fully built, and due to the stacking over time requirement.