The Monkey's Paw

"The devil's toys are offered freely and without mercy."
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Requires attunement by a spellcaster.
During a long rest, an attuned creature may store a number of hit dice up to their Wisdom modifier within the staff. These hit dice do not replenish until expended. An attuned creature may expend a hit dice stored within the staff whenever they cast a spell to accomplish one of the following effects:
• increase the spell save DC by 1.
• roll the hit dice and add the result to the spell