The second book of monsters in the first edition AD&D rule books. Many of the monsters were of limited interest, such as the nilbog and flumph. Perhaps TSR's UK audience found them more useful than us U.S. gamers, or perhaps the Fiend Folio was an early TSR exercise in fluffing out a work that wasn't really fit for publication. It simply did not compare with the original Monster Manual, nor with Monster Manual II. One of its strongest points was the intricate White Dwarf-ish artwork. It is firmly in what I consider the (inferior) "second wave" of AD&D hardcover rules.