Old abyssal had once 30mr shred, which got changed to 40 and 20 over the years, than to 10-25 over levels till it got changed completly from flat reduction to 10% increased MAGIC damage.
If you the mr on Abyssal was just a nice bonus, why are you not fine with
and
? Even the build up items give the whished effect, the finished items give a "nice bonus" too.
gives 15 magicpen,
gives max 10% increased damage at 5s, 2% per second. Note that it jsut says damage, so even physical and truedamage is increased which old abyssal couldn't do. Baiscly with both items you have both cases, old scepter passive which was especialy good vs sqisyhs and the passive that stayed on mask now.
Also
+
+
is cheaper and not especialy better vs magic damage cause beside the botos both items just give ehalth, which is equaly good vs both damage types basicly. Yes, haunting guise with the removal of the flatpen is less attractive now but it still gives penetration and 2-5s in combat should be in, even sometimes more. Also like the reworked scepter good vs even tankier targets.
The nice boni the items give is the now better tankshred of liandrys if the enemy is under 50% health as before and perma grivious wounds, even for manaless champions like katarina and akali. Old morellos gave gv for 8s if the enemy was under 35% health, new gives it for 3s, no matter what. Best is Liandrys passive increases this time basicly for 3 additional seconds.
Yes katarina gets gw for 3s on her ult too but now she can sue it with passive, q, e, items like gunblade or runes.
What can be better for an assassin as gw? Just lb and ahri were kidna good with old morellos as a manaitem and even they skipped it sometimes cause they were made to gunblade champions. Now directly bond to magicpen and not having mana every assassin can use it IF they jsut want to increase their damage. after all you said, the mr was JUST a nice bonus and not cause assassins needed extra tankyness ;). A fizz or ekko can directly go on an adc and with the gw even weaken the protection an adc gets threw the whole lifestealrunes, items and spells/items of the support, so assassination has a higher succesrate. Also the mr mostly didn't even help vs the adc beside maybe the statiksproc, but the health of Morellos and Haunting Guise/Liandrys works fine vs the adcs autos.
Last but not least i want to add that
got also implemented now as a direct dfg replacement (which ludens was intended to be but it also was a pokeitem giving pokechampions even movementspeed. now ludens is fine as a manaitem). Spellbinder gives ap and ms and the active increases both good too for once and assassins should be ok to go in and out once, so the movemetspeed and ap increase for a short time should be realy ok for them (and is better balanced as clicking with dfg just on 1 champion. Also kata increases her damage to everyone basicly and not just to 1 target).