Yes this is an very expenseive build but all you really need to get to is the blood thirster then from then on you will get kills like no other. Once you start getting kills your attack damage gets boosted by the sword of the occult. Oh and remeber even if it says at the top the build gives 365 attack damage, that does not include stacks, with full stacks the build gives you around 505 attack damage.

Kog'maw attack speed is key, he even has a passive just for his attacks speed. The armor pen. runes are for the tanks because I have not included and tank killing weopens in my build do you will have to rely on the runes a bit.

Masteries are 21/0/9 with your utility masteries for kog you really wanna get mana regen because kog does have a mana problem, also in attack be sure to focus on armor pen. and attack speed.

Clarity and flash are amazing choices for kog. Clarity is used because kog is very hard on mana due to his ball of oose and if you enjoy spamming his ult, most people dont like to take the time to let the 8 seconds pass. Flash is usefull because kog doesnt really have any running away abilities and also flashing towards a target could put you just close enough to get your long range ult off so you can get that kill.