Karthus does not need buffs
Karthus ult getting countered by another item does not warrant buffs. That's like buffing jinx because more move speed on boots makes it harder to get caught in her chompers.
Karthus's requiem is situationally good but is and always has been his weakest ability which is meant to be used strategically. The introduction of edge of night does not justify a buff.
Also isn't there now a mana item that can be used after death to deal more damage and heal allies????
The only thing karthus needs in response to edge of night is some excuse to build the more budget friendly
... OH WAIT IT GIVES MR
130 MR
240 ability power vs 80 ability power + 35% mpen
Q = 130 + 30% AP E = 110 (+ 20% AP)
these ratio's aren't great
240 * .3 = 72 +130 = 202
202 * (100/(100+130)) = 87.8260869565
80 * .3 = 24 +130 = 154 154 * (100/(100+.65*(130))) = 83.4688346883
At 130 MR
for 2650 gives karthus Q roughly the same damage as


for 5000g or
for 3800g
edge of night gives no hp and dependence on it is thus perfectly fine for mages if they want to go that direction.
Stacking edge of night with
and
and
will not be practical with additional high damage item against any mage that knows how to build.
some to help compensate