Lullaby

Lullaby has two forms and thus, two appearances. It has its initial and default Flute Form and its Etherious Form.
While in its flute form it is unable to speak, move or take action; and its size is nearly 50 centimeters in length. It has a skull with three eyes on one end, and an oval-like cutout in the skull's brow. On the other end, there are root-like cutouts.

When the flute transforms into its Etherious form, it becomes an extremely tall beast that has three eyes, two legs, two arms and one head. There are many cutouts in its body.
The flute was once just a tool for "murder by curse" at first, which means that it curses the target and causes them to die. After getting into Dark Mage Zeref's hands, he further developed it into a magical flute, called Mass Murder Curse Magic that is capable of putting a death curse on everyone who hears its melody except for the Caster.

(Source: Fairy Tail Wiki)