Problem with Karthus's E passive [GAMEPLAY ISSUE]
There is a bug regarding Karthus's E on patch 7.5, the problem is that the mana-regain passive doesnt work properly when you are utilising your E's aura effect, which makes cs-ing and wave clearing with it more punishable than it should be. As you can see in the youtube link I sent you I killed 5 units, since I'm supposed to gain 48 mana for unit killed and my E costs 78 mana per second I wouldn't even lose mana killing these birds, but I did lose 78 mana and I didn't get it back for some reason. This of course isn't that big of a problem late game since you normally have a lot of mana anyways, but in early game I usually E if I'm about to miss CS and it makes me waste a lot more mana than I whould be, which can lead you to go back earlier and etc. I hope you fix this bug in a near patch!