Headset Unit is an item introduced in v1.0 of Horizon 'Mons. It is meant to be used on Rotom in order to change its form to a Rotom-Vision.
A Headset Unit is crafted by placing Glowstone Dust in the 1st and 3rd slot, Redstone Dust in the 2nd slot, Copper Block in the 4th and 6th slot, and Copper Ingot in the 5th slot.
Using the Headset Unit on a Rotom will change it into a Rotom-Vision