Each weapon (top row) has a support item (bottom row) that allows the weapon to evolve.
To do so you have to have both items equipped and then max out the weapon, following this you open a chest which will evolve the weapon into a more powerful variant.
You must do this with 6 weapons in a single run.
Tips-
- Dont go for this straight away, the further into the game you unlock more items, can reroll, skip and bannish items which makes this far easier.
- Dont rush to pickup chests as soon as they drop, save them until you are ready for evolutions
- there is a grimoire found on the pause screen and main menu which show evolutions
- when upgrading/selecting try to prioritise the weapon. Its the weapon that needs to be maxxed NOT the item
- when selecting/upgrading the choices will show any evo matches for items you already have equipped
- avoid evos that need at least 3 parts (eg guns)
- easiest level to focus on is stage 2, library
- if you really are struggling then late game you can play an endless mode which will give you more time
- late game one of the arcana cards will pull literally anything u can collect straight next to you every two minutes so saves any walking