Instead of making everyone use a potion in lane, how about force Karthus to move (OH NO! MOVING!) to a lane to ult somebody. His ult won't be a tether (like Karma's W), the ult will stay on the target even if they go out of range.
To compensate the range, maybe have it do more damage.
oSEXYPLATYPUSo3 voteshis ult and being able to cast from the grave is what his identity as a champ is...
he is a slow moving , weak laning champ... as it is
JamesTheMage171 votes[{quoted}](name=MrFawknSunshine ,realm=NA,application-id=3ErqAdtq,discussion-id=EmVst4pf,comment-id=0000,timestamp=2017-02-15T23:55:45.053+0000)
his ult and being able to cast from the grave is what his identity as a champ is...
he is a slow moving , weak laning champ... as it is
Weak laning champ. Karthus. Really? Am I reading this right?
oSEXYPLATYPUSo2 votesyea
very weak laning phase
he is super mana dependent he must rush a 
and some get a 
on him... he also has to have a 
he a late game scaling champ
MissingKayo1 votesNo.
QQ more.
Mecha MaIphite2 votesConsidering that he's playing Karthus, I'd imagine he'd be pressing Q a lot.
Stick In Mudd1 votesI think just a karthus rework.
I had also thought of a range to his ult, but then why stop there with the changes? Just remake his entire kit.
