So I rewrote it. It's based on the predicting dice mechanic that the Divination School wizard has, which is one of my favourite underutilized ideas in 5e. It's definitely overpowered, but I haven't decided where to put it or what to do with it, so that's ok.
Probably what I'd do if someone wanted to use this is have a lower level 'Thief of Three Fates' or even a 'Thief of One Fate' ability, and make the sucker's get the prereqs. But I dunno.
Five times per day, you may ‘steal’ another character’s d20 roll. Record the result and add it to your pool. You can give them a replacement result from your pool, or just let them roll a new d20 to replace the one you stole. Whenever you roll a d20 for a save, ability check, or attack, you may replace the result with a roll from your pool. Using a number from your pool for either of these purposes removes it from your pool.