A Potential Solution To Karthus vs. Edge of Night
#Requiem
##Active: Karthus channels for 3 seconds, marking every enemy for death upon successfully completing the channel. Enemies marked for death take magic damage after 3 seconds, with a beam of death hanging over their heads for the duration. The mark's application can be blocked by spell shields, but marked enemies cannot block the damage by any means other than invulnerability.
What does this change?: Since the beam-of-death effect would be moved to the end of the channel and would take the form of a mark, enemies wouldn't be able to spell shield Karth's ult unless they see him channeling. Effectively, global targets would not be able spell shield the damage if they get taken by surprise, nor would untargetability spells like Playful Trickster or Sanguine Pool block the damage anymore (Zhonya's still would, though, due to its invulnerability).
Could this be improved?: Absolutely. The channel could perhaps be removed, since the counterplay tied to it is situational at best (Karth can always hide in a bush to ult for exactly the same effect, and his channel is uncounterable while he's dead), and it would bring the time from cast to damage back to 3 seconds. Personally, I'd like his ult to be taken to a new extreme, and make it a death mark that auto-executes every champion on the map below a certain threshold at the end of its duration, regardless of spell or damage immunity. Ultimately, though, these are likely large buffs, and the core proposal in this thread is mainly to make Karth's ult less weak versus much cheaper spell counters.
Also, Riot, if you're reading this, please make Karthus's ult globally deliver one of his quotes. It makes no sense for champions like Kalista and Kled to have global quotes and not Karthus, the king of global ultimates, especially when his ult already has a sound effect and all.