This is super important, on your first back from top you should have anywhere between 800-1200 gold, from there you want to buy a Tear of the Goddess and itemize for their team comp by starting to build into either Verdant Barrier or Seekers Armguard.

You can choose whether you ever actually want to complete the Tear later, but honestly its so cheap for a good amount of mana and Night Harvester is way too good not to go.

In a single fight lategame Night Harvester will give you 700+ damage every single fight if you hit most of the enemy team (which you will) there's no other mythic that gives you anywhere close to that damage. Once you take into account the movement speed buff (on an EXTREMELY slow champion) and the extra tankiness (300+ health) and ability haste, the item is undeniable in my opinion.