If so, does he build into the seraph's? Does it replace the deathcap or should I get that as well? Normally I go RoA, boots, Rylai's, Seraph's, Abyssal, and Zhonya's. Also what masteries does he use? I've been using deathfire's.
Rolling Girl2 votesDeathfire is his best option for masteries, I think. An early tear and then your other AP goodies before building seraph's at 750 mana is generally a good idea. That being said, poor Karthus has been kind of weak for a while now : (
Tworie2 votesYou can always wait until it maxes out before buying a seraphs and just get a Deathcap instead.
Titanium701 votesI don't play Karthus right now. However IF someone would make me play him I'd go for the META-AP route.
into what ever usefull 
.
Everything else is just far to slow for the current META.
Poske1 votesBassicly this
You want your ultimate to halve your enemies hp around minute 20 way better then going full mana regen karthus
Viktor Von Lazer1 votesI'd Always build tear on Karthus, He needs the mana for his defile. You can skip out on RoA as both tear and RoA might be too slow. Haunting Guise + Sorc boots depending on how you see the game going can grab you some early cheesy kills because your ult will effectively do true damage to your targets early game when everyone is building core damage items. Build depends largely on style of play. Are you looking to grab cheesy kills around the map while constantly shoving in mid not giving your opponent time to roam and threatening first tower if they do? Then I'd just go full pen early (sorc + haunting guise for true damage) get a rylais, upgrade the haunting guise after rylais for synergy and build any other AP items i feel necessary after that, (Void, zhonyas, deathcap) Taking TP as my summoner so i can always push mid. Are you looking to scale and duel it out in lane? well RoA is better for that and taking exhaust is best summoner in that case.
Rylais abyssal and zhonyas is too defensive a build. You have to remove one of them depending on the enemy team's damage type and add either Deathcap or Void depending on their build. (a lot of MR void, a lot of health - liandry's - not much defensive items - deathcap)
Chønklord1 votesDoesn't matter anymore. Have you seen the new items being made? 8 sec cd spell shield for mages. That plus new ga for ad's and eon:
Karthus new build:
(Boots of mobility so he can run away faster whilst crying).
