A magic item of this sort appears to be any one of the
small, rather weak miscellaneous magic items which are more frequently
found - typically one with a limited number of usages, although
this is not always true.

If the possessor commands, the item
will suddenly alter itself to become any other miscellaneous magic
item except an artifact or relic. This enables the possessor to command
the item to become some form of Horn of Valhalla, for instance.
The limitations on the item are these: No form can ever be repeated. If
the item commanded into being is normally destroyed in usage, the
anything item will likewise be destroyed after that use. If the item duplicated
has a short-term, single-usage effect (such as a horn or
drums), then the anything item will return to its original form after one
use of that power. If the item has a long-term effect (such as a helm of
underwater action), the dweomer of the anything item will last for one
day (24 hours). In any case, the item will disappear after three uses.