I think that because the mana level of Karthus is extremely high, Tear of the Goddess is a awesome item because if you have extra time late game, just spam Q in base and you gain max mana and AP based on 3% of your max mana. This meaning, for every 100 Q's used with a 1 second cooldown, you get 3 AP.
Romans VI XXIII2 votesif you have extra time late game,
If you don't have it fully stacked by late game you are doing it wrong
Ralanr1 votesI always get 
staff with Kathus. Though I also like getting 
with it.
Death by Glamour1 votesWell its a terrible late game item ;D
Usually i tend to go tear into roa or rylias depending on how mobile the enemy team is.
Joe Defense1 votesI rly like the feedback I will be getting because I am a fresh karthus player and am going to buy because of cheapness. Need some suggestions to play him. Did my second game Karthus this morning.