Hello,
my question is why spellcrits like karthus q or soraka q cant be amplified by Crit-Damage-Runes or the Infinity Edge. I would really like to build the Infinity Edge on my Karthus (or at least use some Crit Dmg runes to be less trolly).
If there is a reason why this doesnt work at the moment, please tell me. 
OverpoweredSoap4 votesI get that buying 
would be bad for Karthus, but let's not forget that since his Q _technically _ crits when it hits a solo target, it's damage is reduced by 
.
At least that's the case for 
's E.
EndlessSorcerer1 votesEven if they could, I'm not sure why you would want to. It's definitely not the most efficient rune-wise and Magic Penetration would pretty much always outdamage it.
CC Dathalion1 voteswell i think the late Game Damage could be slightly higher with the Crit Dmg Runes.
EndlessSorcerer1 votesThe majority of Karthus' reliable damage is due to his Defile and his Requiem; hitting isolated targets with Lay Wastes is unreliable and there would be such a small difference between the damage increase on the isolated Lay Wastes (between crit damage and magic pen) that you would take magic pen anyways because it increases the damage on all of your abilities.
MisterHealYoGirl1 votesActually, magic pen marks would increase your damage by about 6%, while an IE would boost your Q damage by 50%. However, it's also the most expensive item in the game, and it has no synergy with the rest of your kit at all.
EndlessSorcerer1 votesAn IE would boost an isolated Q's damage by 25% (if it would have dealt 200 damage, it would then deal 250) raw damage before mitigation. Crit damage runes would not affect non-isolated Qs, Requiem, and Defile.
MisterHealYoGirl1 votesOh, it still wouldn't be a good item on him even if it did work. I"m not arguing that.
EndlessSorcerer1 votesActually, I was initially comparing crit damage runes to magic pen runes. I wasn't even going to bother touching IE for obvious reasons.
Stiv12341 votesNo, it doesnt. It is just there for clarity.
CC Dathalion1 votes? What do you mean?
DrCyanide1 votes reduces the damage it deals, because that "clarity" is the same text used as Crits. Riot thought it should be abundantly clear that the item doesn't reduce actual crits, just attacks that have special text formatting...