Shadow (also called Darkness Crystal) is the primary permanent-upgrade currency on the Underworld route. It drops reliably from boss kills and treasure rooms in Tartarus-side runs, with small amounts appearing from regular Underworld rooms as well. It is entirely absent from surface-side runs, making alternating between both routes essential for steady supply.

• Upgrading and unlocking Arcana Cards on the board (primary use — the main driver of build strength)
• Serving as a catalyst ingredient in mid-tier Cauldron Incantation recipes
• Supplementary cost in some hub permanent-upgrade menus

Shadow is the backbone of Arcana progression and should be spent as soon as a meaningful upgrade is available — stockpiling it provides no compound benefit. The optimal spend cycle is "collect 20+ Shadow, then immediately invest it all into the Arcana board." Players who focus exclusively on surface runs will find their Arcana growth stalling; balancing both routes keeps Shadow income healthy and progression smooth.