PSA: Psi Beetle Brain in a Jar location

Recently found this item and wanted to post its location, since there's scant info about it on the wiki and I've seen several users in other forums asking about it. It's found on a character called Lunatic Beetlebrainer (see screenshot) who spawns randomly with a group of other Lunatics in Upper Underrail. I encountered them on Dominating; I can't confirm if the random event occurs on lower difficulties.