Wondrous Item, very rare (requires attunement)
This bag has an extra-dimensional interior space that traps any undead creatures who also have the Incorporeal Movement feature. The bag weighs 15 pounds, and can hold a number of entities equal to d12 + the user's Spellcasting ability modifier. Releasing a specified creature from the bag requires an action, and a successful DC 15 Arcana check. On a failure you release a random entity determined by the DM.
If the bag is overloaded, pierced, or torn, it ruptures and is destroyed, and the entities are released back into the Material Plane.
Notes: Utility, Container
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