While playing mid. with Karthus it will be very posible to return to base after lvl 6, this guide of Karthus wont help you in the bigining, becouse the only items for starting in this guide are for mana and helth regen. after lvl 6 Kartus becomes OP if u get the boots and the Rog of Ages. when u get to lvl 11 and buy the Void Staf which gives him AP and Magic Penetration his ulti will deal SUPER OP DMG. However after lvl 11 all of the champ. on the arena becomes more stronger, so you will need more power for more kills, take Rabadon's Deathcap this will incrice your AP and combined with the Magic Penetration he will Rull. The next other items I picked becouse they bouth give AP and give you more HELTH and ARMOUR which Kartus needs in order to survive after 18 lvl.

I actualy havent play Kartus Bot or Top with Players ... I played bot with him once in Co Op game becouse my team was full of noobs. Kartus was doing fine bot but u may want to get the Cataclyst of Protector before the Summoners boots, becouse dealing with 2 guys at once will drop more HP and Mana.