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.
Karthus' ulti is probably the funniest in the game since an enemy thinks he gets away and pow he dies instead. karthus can kill enemies with his other spells to but it is important to get some cool down reduction so his ulti can be used as often as possible.
Cool down reduction
good health and mana
high health and mana regen
almost no armor and magic resist
low base damage