Karthus E should have a built in Rylai’s slow attached
Karthus E is similar to Morde passive where there is a ring around the champion that deals damage to anyone near, but Morde buys a Rylai’s to keep people inside the ring and he is tanky. Karthus E does the same thing where it provides damage and nothing else, but Karthus is squishy and needs to get close to deal moderate damage for no benefit.
By giving Karthus E a Rylai’s slow attached it allows his E to be a more fulfilling ability and one that actually has a purpose instead of being in melee range for a poor version of Morde passive.
Karthus W doesn’t provide enough of a slow because it decays too quick. Also by providing a slow to Karthus E, it allows him to hit his Q more easier rather than missing many and all his DPS going down the drain.
Karthus with the E slow will actually be able to punish melee champions again back in the old days where Rylai’s was core on him.