User blog comment:Nelraye/Lore Discussion: Riot, what happened?/@comment-5116121-20140221204513/@comment-24501229-20140222081539

For Thresh and Kha'Zix, there's a huge question on how the fields of justice function. Champions revive after they are killed. It stands to reason that their souls are not lost during this process. The wiki article on "death" says:

"When a champion is summoned to the Fields of Justice, their real, physical form is summoned there, meaning that the champions on the Fields of Justice, do, in fact die upon player death. When killed, it seems as though they are transported back to their respective summoning chambers, brought back to life, and subsequently returned to the Field of Justice to continue fighting."

So it seems highly impossible that Thresh is collecting any souls at all (let's ignore the fact that everything drops souls in the game itself). And Kha'Zix wouldn't be able to consume much of a champion before they are recalled and revived, else there wouldn't be anything left to revive.