I've lost at least a sona and janna game to adc karthus so far, probably more but those recently stuck out as games we got analed botlane.

It might just be luck or that karthus has high numbers and is strong in general.

But also the matchup feels hard for spellthiefs supports because he has really long range and is hard to poke, sort of like seraphine. It's a lot easier to Q short range adcs than adc karthus. But despite being long ranged he's really strong all in? His Q just does a lot of damage early too. So idk you can't outrange him to poke and if you get close you lose all in. Also in the all in your adc has the ghost summoner spell (does nothing) and karthus has exhaust (slows 30% and reduces dmg by 35% for 3 seconds) so it's hard to trade a kill back on him.

Also it's kinda awkward to take an mr rune if you'll mostly take physical dmg but maybe whatever we just take the rune for lane.

Idk it might just have been bad or unlucky games or one of those picks that would get nerfed if more ppl played it.