Pretty much went the entire campaign and cleared Echo Knight 5 times on top of all pestilence bosses before I even discovered the alchemist. There was always that glowing door but its access point was not visible to the player from the vantage point of Mariner’s Keep, so I just bumped into the ladder to come down and walk into it through experimentation.
Unlike Gordon, who you get the wife prompting you to go chase him and there’s a marker in the map, no one even prompts you to look into the alchemist.
And it’s a shame because it is bar none the COOLEST EFFING QUEST IN THE GAME. Easily missable for some reason. That mushroom trip was gorgeous and fun.