The only bad thing with this build is the mana problem, if u don't want this mana problem u can begin with mana regen instead of boots. Also you can change the health runes to mana regen runes. You should also take the mana buff as often as you can

Pros:
Great AP burst damage.
One of the best harasser.
Can still take down tanks with Bio-Arcane Barrage.

Cons:
Low-healthed
Very Slow.
Often Focused.

Lore :

When the prophet Malzahar was reborn in Icathia, he was led there by an ominous voice which thereafter anchored itself to his psyche. From within, this voice bestowed upon him terrible purpose, and though Malzahar was no longer tormented by its call, the voice did not cease its unrelenting summons. This baleful beacon's gentle flicker -- now fastened to Runeterra -- drew forth a putrid beast that ambled across a threshold it did not understand, widening a fissure between the spaces which were never meant to meet. There amongst the haunting ruins of Icathia, Kog'Maw manifested in Valoran with unsettling curiosity. The spark which led him to Runeterra teased him still, urging him gently towards Malzahar. It also encouraged him to familiarize himself with his new environment, to the stark horror of everything he encountered on his journey.

The enchanting colors and aromas of Runeterra intoxicated Kog'Maw, and he explored the fruits of the strange world the only way he knew how: by devouring them. At first he sampled only the wild flora and fauna he happened across. As he traversed the parched Tempest Flats, however, he came upon a tribe of nomads. Seemingly unhampered by conventional rules of physics, Kog'Maw consumed every nomad and any obstacles they put in his way, amounting to many times his own mass and volume. The most composed of his victims may have had time to wonder if this was due to the caustic enzymes which stung the ground as they dripped from his gaping mouth, although such musings were abruptly concluded. When his wake of catastrophe reached the Institute of War, and enthusiastic Malzahar greeted him with an enticing prospect: taste the best Runeterra could offer...on the Fields of Justice.

'If that's just hungry, I don't want to see angry.' --Tryndamere, the Barbarian King

Champion Spotlight:
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I like the AP Kog'Maw. But still AD or Hybrid Kog'Maw is both strong. For me i think AP is more funny and i enjoy playing it. But AD is the stronger one i think. I think it's just what you like to play more. So if you enjoyed this build and if it seems intresting, give it a try! thank you for reading :)