Torrona's Box is unlocked by evolving 6 weapons at once in a run.
(With the
Tides of Foscari dlc, Torrona's Box is a level item in Abyss Foscari - If you go here, just put on the Arcana
Mad Groove that summons all items to you when you start, then level it up as usual.)
This item is cursed. It is incredibly useful for the first 8 levels, but at level 9 it adds 100% curse. Try to make this last level the final upgrade of your equipped items to avoid being overrun.
If you haven't unlocked
it will unlock after the 20 minute mark as well with this item maxed out, regardless of if you had the curse a full 20 minutes.