When Arnvid's unseen limb is cast, the wizard causes an invisible limb (arm, hand, leg, or foot) to come into being. This limb can replace a missing limb or it can be used to create an extra one. The invisible limb functions exactly as a normal limb, except that at the option of the recipient of the spell, parts of it can become immaterial so as to pass through solid objects. For example, the limb could be used to uncork a potion inside a closed chest and dump the bottle out, but it could not for example, to add a second shield arm in the middle of the recipient's back. The invisible limb has no hit points or Armor Class as such, and it cannot be harmed unless it is dispelled.
The material component of the spell is the tail of a lizard (any type that regenerates lost body parts). This is touched to the place on the body where the limb is to be restored.
Notes: Very rare. (Updated from Dragon Magazine.)