Root Beer is a beverage made from a plant which can be found growing on Monkey Island.
It is also known among Voodoo Practitioners to be a powerful weapon used to destroy ghosts. And it tastes pretty good with some vanilla ice cream.
Guybrush gave chase and on arrival on the island, used the mixture to destroy a number of LeChucks ghost crew before confronting him in the church.
LeChuck knocked the mixture out of his hands and only by chance Threepwood found another bottle of Root Beer after a punch from LeChuck sent him crashing into the Grog Machine at Stan's Used Ship Emporium, which he shook up and sprayed at the ghost pirate. LeChucks spirit form was destroyed in a display that looked like fireworks.
LeChuck was resurrected in his rotting corpse and once more a confrontation between and Guybrush Threepwood took place. Guybrush found some Root Beer from another Grog Machine under Dinky Island and attempted to use it in the same way he had a year before, but found that it was ineffective, as LeChuck was not a ghost any more.
A flat bottle was found kept inside the Monkey Casket of Kauai. Threepwood needed it in order to complete the creation of his Cursed Cutlass of Kaflu. First he had to make the beer fizzy again. He did so by adding a Breath Mint. Soon after, he dropped the bottle and was forced to find some more. He created some makeshift root beer from Chuck's Roots, some lite-grog and another mint.