Quick answer: Boss Souls are crafting components. Hold them until the Craft tab shows a recipe that supports the current class and the rest of the route.
How recipes appear
The inventory Craft tab surfaces recipes when the relevant materials are available. Some combinations remain hidden until a key component is collected.
When to craft
- Craft for the run you are playing, not a hypothetical later build.
- Coordinate through the shared Stash before consuming a party-critical material.
- Prefer an immediate route-solving upgrade over a marginal sidegrade.
Common mistake
Do not assume crafted gear is permanent account progression. Treat materials and equipment as part of the current run unless the live UI explicitly says otherwise.