TSR UK Ltd.
TSR hired Turnbull to edit a second book of monsters, which he named the Fiend Folio, after "The Fiend Factory" monster column from White Dwarf, where many of the book's monsters first appeared. : 12 Games Workshop negotiated a deal with TSR in 1979 to put this book together by compiling content from the "Fiend Factory" and other British publications. : 45 It was published in 1981 by TSR UK Ltd. Like its companion volume the Monster Manual, the Fiend Folio provided a listing of creatures that player characters could encounter as part of the first edition of the Advanced Dungeons & Dragons game system. It was the pages of the Fiend Folio that introduced a wider audience to the githyanki, the grell and many others.
TSR founded TSR UK in 1980, and Turmbull led this new undertaking. : 13 In his role as managing director of TSR UK Ltd. he oversaw the publication of a range of titles, including his own "Underwater" modules in conjunction with Dave J. Browne: The Sinister Secret of Saltmarsh, Danger at Dunwater, and The Final Enemy. These have been described as having "Probably one of the best storylines of any module series." These early TSR UK adventures U1–U3 (1981–1983) were incorporated into the Greyhawk setting after the publication of the World of Greyhawk Fantasy Game Setting (1980). : 24 The Sinister Secret of Saltmarsh was elected the 27th greatest D&D adventure of all time by the official D&D magazine Dragon.
TSR UK Ltd. also produced the "UK" series of modules: UK1 Beyond the Crystal Cave by Dave Brown, Tom Kirby, and Graeme Morris; UK2 The Sentinel by Graeme Morris; UK3 The Gauntlet by Graeme Morris; UK4 When a Star Falls by Graeme Morris; UK5 Eye of the Serpent by Graeme Morris; UK6 All That Glitters... by Jim Bambra; UK7 Dark Clouds Gather by Jim Bambra and Phil Gallagher. Some of these modules had their origins as the "GamesFair" tournament adventures. Don Turnbull was also the publisher of the UK based RPG magazine Imagine.