You know what Karthus really needs? A small QoL buff with huge results.

When I say QoL buff, I mean this:

#Make the quote for Karthus ult play globally.

We don't like hearing Illaoi whisper when recalling all across the map, and we don't like hearing Kalista's gibberish-like babble (Seriously, what the hell does "you shall not hand" even mean anyway? Read a book, lady.) But hey, I guess it's important. But not one when Karthus ults? In the words of Taric, "outrageous."

When Karthus uses his ultimate, he sings a requiem, calling down death to smite down his enemies all in one moment. Wherever they are, wherever they go, death follows. And that impending doom as they cry out in fear and are struck down, their souls torn from their bodies. And Karthus yells the coolest things like "Surrender!" and "Sing with your final breath!"

I think this will both make Karthus feel more foreboding and a threat without changing any numbers while also creating the theme that Karthus sings the word of death, and every living and undead thing hears his call and quivers in fear.

(Also, who doesn't want to hear Pentakill Karthus scream "I am the lightbringer!!!" in that awesome rock voice?)