Abyssal Scepter

when dealing 1000 damage to a champion with the normal MR of 30 you can calculate the damage with the following equation

(1000) * (100/(100+(30))) = 769.230769231

costs 2750 and gives 60 AP and 60 MR and 10% cdr but I'm going to focus on the damage which increases your magic damage by 10% as well as everyone elses.

But before that I'd like to point out that whether or not its changes have been a buff or nerf is irrelevant as times have changed and all that matters is whether or not its a good buy.

so we have (1000) * (100/(100+(30))) = 769.230769231

lets penetrate 15 MR with and see what happens

(1000) * (100/(100+(30-15))) = 869.565217391

so lets see how much of an increase that is.

869.565217391/769.230769231 = 1.13043478261 or 13% increase. Against a ranged champion with no bonus MR abyssal's 10% damage increase is probably nothing special.

But melee champs tend to have more MR, lets say 60. So lets see the difference there

(1000) * (100/(100+(60))) = 625

(1000) * (100/(100+(60-15))) = 689.655172414

625/689.655172414 = 1.10344827586 or a 10.3% increase

ok so now its a lot closer when the opponent has more MR.

If they have at least 60MR you could get or and get the same damage increase if you are dealing 1000 damage but really it shouldn't matter if you are dealing a different amount since all that's changing is the damage modifier. Lets check real quick.

(400) * (100/(100+(60))) = 250

(400) * (100/(100+(60-15))) = 275.862068966

275.862068966/250 = 1.10344827586 so same thing

Now you may still be asking yourself if it was a buff or a nerf. I have a simple answer for this now that I think about it.

+ now effectively penetrates more MR against tankier targets at least and maybe squishy ones as well but I haven't really done the math since I'm not really worried about mathcrafting for squishy targets, I just do that by eye usually and I've already types a lot

TL;DR

it appears to have some purpose, particularly against anyone with at least 60 MR