Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
U max e for the reason that it is very hard to land ur q. And it is actually fun to watch your opponent die slowly while u die faster.
Try not to fight due to karthus is very weak at 1v1.
Tell your jungler to gank more.
Don't push the lane, unless u r fed.
Get assist or kills from ulting in order to get fed.
***DONT GET KILLED***
In teamfight, try to flash in or get into the middle, activate Seraph's Embrace or zhonyas hourglass, exhaust their most fed person, stay alive.
Once u die keep q and w, activate ultimateat the last posible second.
Karthus is very devastating when enemy is doing baron/ dragon, due to the fact that they are grouped up.
U can do absurd dmg.