The witch's cauldron
The cauldron turns gathered materials into potions, and potions are the only pre-fight choice in the game that is not permanent. Everything else about your character is a build decision you live with; a potion is a decision about one specific fight, which is why the cauldron opens around the point where ordinary upgrades stop being the answer to a wall.
How it works
Recipes consume materials and produce potions you choose before a fight rather than during it. Because combat resolves from what you brought, a potion is not a panic button but part of the loadout, and its value is entirely a function of the enemy: mitigation against something that spikes, offense against something you are almost outpacing.
Best practice
Brew against a named obstacle. A potion is worth the most when it converts a fight from nearly winnable to reliably winnable, which means the right time to open the cauldron is immediately after something has stopped you, with that specific fight in mind.
The common mistake
Stockpiling one of everything. Materials are finite and a shelf of unused potions is a shelf of upgrades you did not make. Brew what the next wall needs, use it, and let the rest wait until there is a reason.