Financial Wizards are NPCs that only appear in Deadman mode, located in banks that are in safe zones. Players can give them ten items that they wish to keep safe; when the 10 items are given to the Financial Wizards, they are securely stored in the Safe Deposit Box and cannot be used.
The Safe Deposit Box, accessed by visiting a Financial Wizard.
You cannot store untradeable items. Stackable items such as bank notes, runes, arrows, bolts, etc. will not count as one item. For example, 10 air runes will count as 10 items.