Kog'Maw
Artillery of the Living
Lane harassment and consistent damage output.
Precision
Improved attacks and sustained damage
Lethal TempoAttacking an enemy champion grants you Attack Speed, up to 6 stacks. At max stacks, deal
bonus adaptive damage On-Attack.
Absorb LifeKilling a target heals you.
Legend: AlacrityTakedowns on enemies grant permanent Attack Speed.
Coup de GraceDeal more damage to low health enemy champions.
Sorcery
Empowered abilities and resource manipulation
Manaflow BandHitting an enemy champion with an ability permanently increases your maximum mana by 25, up to 250 mana.
After reaching 250 bonus mana, restore 1% of your missing mana every 5 seconds.
Gathering StormGain increasing amounts of AD or AP,
adaptive over the course of the game.
Adaptive Force
+9
Adaptive Force Adaptive Force
+9
Adaptive Force 
Health Scaling
+10-180 Health (based on level)
Why Lethal Tempo
Kog is an artillery AD Carry, meaning his damage is directly related to the number of auto attacks he can get off in a fight and Lethal Tempo drastic ups that amount.
Why Absorb Life
Overheal allows you to absorb and sustain through lane poke so that you're able to stay healthy for trades in the mid game.
Why Legend: Alacrity
Legend Alacrity further empowers your ability to pump out damage from auto attacks quickly. Thanks to the removal of Attack Speed cap from Lethal Tempo, you're able to build multiple Attack Speed items without capping out.
Why Coup de Grace
Coup De Grace is the go to rune choice for reliable damage, espeically in the bot lane when looking to kill squishy targets.
WhyManaflow Band
Manaflow Band is excellent for early lane phase harassment with [Q] and [E].
WhyGathering Storm
Gathering Storm is your late game insurance policy. The extra Attack Damage when multiplied from Lethal Tempo makes you the biggest threat in the game if you're able to hit 40+ mins.