love it

no defensive items or utility

First off all the masteries. of course teleport works, but is this build for mid or not? and kog is not the best backdoorer, good but not the best(like sivir/yi). so flash/exhaust are better choices
Second about runes.
Why aspeed marks? mpen and arpen is by far better(even tho almost all kog's dmg is magic)
even auto attack(bio arcane and madreds) so get mpen.
seals and glyphs are good and arpen? i say get mpen. i do see your ad build. but since even madreds is magic those are better. and kog scales much better with ap than ad.
Third the build.
Well first of all you will be shquishy almsot the whole game(whole if short or unfed)
and uber easy to kill mid if against a nuke. but if you can handle that its okay.
Berserker boots is not really necessary and sorceres can be better,but you have arpen so i get the idea.
Rush madreds is good but gives you a very bad early game due to a costy item and not the best recepie. and madreds is better in high level. since the passive does more the more hp they have.
I dont get the phantom dancer. what about sword of the divine? 1k cheaper and more aspeed + active + magic dmg. much better. or malady. 1k cheaper aspeed and mpen
And i dont see why you get last whisper. no aspeed and arpen is kiiinda useless
masteries is also a little bad. 21 in offence is not needed. but its good enough. but the crit dmg is really waste.

Mvh
Vetle158

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I don't see how the masteries are "no good". They are not flawless but they work for kog'maw

no good masteries

i tried it and it really worked,

huzaa

Not bad... +1