Karthus' ultimate is too binary, and is in need of more baseline functionality

As it stands, Requiem is not a very good ultimate in League's current state. While the damage it can deal is potent, it's far too easily countered by and so on, including high MR items. Various Champions can also block, avoid or heavily reduce it, further reducing its usefulness. This leads to high unreliability, which is bad since Karthus packs all his power into Requiem and can't be too strong elsewhere because of it.

On its own, Requiem is impractical since it only does damage and has a very long cooldown. While you can add a slow with , that just isn't enough and doesn't help with Requiem's "easy to avoid" problem. But rather than try to deal with the evasion/blocking issue, which would make Requiem mechanically overpowered, I think that adding some new baseline functionality would be a better option.

To put forth an example, Mephisto in Heroes of the Storm has an ultimate that is basically Karthus' Requiem, but he adds on a reveal and slow effect onto it. Mephisto's version doesn't do as much damage, but the reveal and slow are strong utility, allowing his teammates to track down enemies or catch up to fleeing opponents.

Adding either the slow or reveal onto Requiem would be a big help, extending its effect to more than just raw damage. Raw damage abilities on such a long cooldown are not very viable, and I doubt Riot would've make Requiem like this if Karthus were made today. Between the two effects, I'd go with the baseline slow so that Karthus and Twisted Fate don't have a niche-clash. Adding a slow would allow Karthus to help allies secure kills early on, and when used for teamfights the slow can be very useful to let teammates land AoE abilities. Of course, the wind-up will still cause it to be counterable, but its impact when it successfully hits would become more pronounced. Also, having the slow as a baseline effect means a Karthus player would be able to forgo taking and pick up something else, unless he wants the rest of his kit to have slows as well.

While abilities and ultimates that are pure damage are not uncommon, and certainly not unviable, Requiem has that long wind-up and very long cooldown that make its singular function not work that well. With all the new mechanics that have been added over the past few years, it's now easier than ever to avoid it. Karthus hasn't been strong in a long while, primarily because adjusting his numbers is not a good way to adjust his balance. Adding a slow or reveal onto his ultimate would give him some much-needed utility to push him back into more team compositions.

And even if Karthus might be in an okay spot right now (thanks for pointing that out, Ratpie), adding in a baseline slow onto Requiem at some point could allow Riot to have more balance levers to pull.