This is a smallish accordion-like instrument emblazoned with an abalone skull. Which is to say a decorative representation of a skull carved from abalone, not the skull of an abalone. I don't think those things even have skulls.
You must have an accordion in your inventory to cast Accordion Thief buffs.
Any Accordion Thief buff cast while a Calavera concertina is the best accordion in your inventory will have a duration of 7 turns per casting (unless a stolen accordion is equipped).
References
This item is a reference to the calavera, a skull-shaped sugar sculpture (whose name means 'skull') used to help celebrate the Day of the Dead.
Being a mollusk, an abalone does not have a skull.
