A Fetid Alcowalker is found along the path to Laocoon. You will need the password and cannot use the portal from an alliance hall, as the entrance to him is just after the spider with the key but before the first trial.
Before the laocoon and Fetid Alcowalker can be reached, you must go N, U, N, E from Sigil recall and say something to the white-maned man. The druid will tell you about her treating him, but she ran short on money to complete it.
It is up to you to spot her ~1 million gold. Make sure to give the DRUID 1m and not the white-maned man. She will give you a parchment with the password good for 72 hours.
Head down to Palace of Diocletian and make your way to the catacombs. From the entrance, go 4n w n e 3w nw sw s 2w n until you reach a barrel-chested cultist deep within it.You'll know you're going the right way when the volume of monsters drastically decrease. Sayto blind cultist <password you just received> to gain entry into the beginning of the Laocoon's area.
Scan around this area and look into rooms before moving for a hidden monster named Black Fijuez. The spider can both inflict bite paralysis and cast spells. The spider does not have a lot of health and is easier than both bosses. After killing it, you'll get a key. Climb down the wall and unlock the door deeper into the catacombs.
The Fetid Alcowalker will be in the next available south path, and is in the first east room.
This boss has an instant kill mechanic. If you are sober when you enter combat with the Fetid Alcowalker, he will instantly kill you. So make sure to bring alcohol and drink it while fighting him. He will also make you flee each round.
Prep: You will need extra alcohol on you, battle march or fearless to avoid the fears (can also room shield yourself in). I recommend bringing invisibility for all bosses.
Fight: As you fight the Fetid Alcowalker, spell him up and remember to drink alcohol. He will occasionally famine you and drain nutrients, but he does not hit hard. If you become sober during the fight, you will die in CPK.
Drops: Marbleskin tail and aegis.