If you make it that far before sealing your fate, the bartender will say, "Be sure you handle this with care; this is the finest I have." From here, you may do one of two things: Say, word for word, "I overestimated my fortitude, and I bid you good eve.". If the barkeep nods, you may leave the door you entered, unharmed and with nothing gained and nothing lost (except the time spent inside).
Or you can go on. You will be given a glass with a seven-sided rim, with each side twisting ever so delicately around the basin until forming a sleek and simple handle. You will also receive a very, very, very special absinthe spoon, in the shape of a key; the holes at the key's top serve as the draining point for the alcohol to pour over the sugar cube. And, of course, an unmarked bottle, stripped long ago of its label, scraps of paper sticking to its sides, covered in the rot of the decades past.
The spoon is completely flat, but has two distinct sides: one with a groove along the shaft of the key, and one without. Turn the shaft down, so its groove will be face down. If you attempt this face up, your absinthe will taste foul, your nose will burn, and your eyes will shrivel in their sockets with unspeakable horrors not of this world. Now, if your spoon is the right way up, begin preparing the absinthe as one would (put the sugar on the spoon, and pour the alcohol over so it gains its color and "special qualities").
Say "cheers" to your friend, the barkeep, and bottoms up. If you don't, the absinthe will burn every innard it touches with the power and pain of sulfuric acid.