Mushroom beer is a alcoholic beverage made by mixing juice squeezed from mushrooms with a little bit of yeast and then letting it ferment. Mushroom beer is the most common spirit in the underworld due to mushrooms being one of the few common underworld flora that can even remotely be fermeneted.
The first batches made by the Tei'kaliath were rather unappetizing at first, although extreme alcoholics (or rather Brorn) consumed these early trial batches with great gusto. Eventually the Tei'kaliath did manage to make a drinkable mushroom beer.
The Tei'kaliath have since discontinued the making of mushroom beer since coming to the surface and now make wine instead.