Talk:Karthus/@comment-1920011-20160112094406/@comment-4091261-20160112160706

Cooldown reduction is also moderately inefficient due to having no cooldown. Then again, it truly is the only way to utilize better. So cdr is definitely something to consider, but there needs to be a bit more to boost damage output.

Due to having immense base damage, getting a large amount of magic penetration would be worthwhile. In fact, getting Magic Penetration Quintessences are actually better than getting  for a rank 1  onwards.

Having a strong mix of Magic penetration and Cooldown reduction is powerful on. I personally just go full flat CDR runes with the traditional build since the bullying potential is insane for early game. I also find that the magic penetration items are more useful than the cdr items are for since they grant more survivability in comparison.