Hmmmm... Thinking something like:
Upon Mega Cho's entry to the Rift, he will roar mightily (think: Ogre Roar) to announce his presence, with a message/quest instructing the player to kill Mega Cho TWENTY (20) times
PASSIVE: World Eater: 3 part passive:
*Mega Cho deals 20/50/80 (scales with Feast Stacks) magic damage over 3 seconds to enemies he is moving on (like an infinite Alistar passive), and below 50% HP, his Attack Speed increases by 50/80/100% and he gains 20/30/45% Life Steal.
Mega Cho receives Feast at level 1 AND gains special power-ups based on his Feast Stacks. *
He gains extra Health and size as normal:
1-6 Stacks: +5 Bonus Armor/MR as well as Health bonuses.
7-13 Stacks: 20% extra Base Regens (HP and Mana; max total of 140%) This bonus doubles when below 20/40/60% max health
13-20: Passives scale. Each Feast Stack grants larger spell range and radius (Area of Impact).
20-30: Audio plays and announcement shown, hinting Mega Cho hitting 20 Stacks, and gives 1% Armor/Magic Penetration per stack (Max total of 7% each)
30-40: Mega Cho's Passives scale again. He gains 3% Armor/Magic Penetration per Stack, capping at 30%
40-50: All Spells gain 30 Base Damage, capping at 270 extra damage.
50: The game ends. Mega Cho will move on to devour biomes, cities, and, eventually, the world.
NOTE: He is very big, and can only use Feast on Towers, Large/Epic Monsters, and Champions.
%HP and true damage is capped at 200/400/600 Health, and he gains. He's that big.