Volume IV of the Arduin Grimoire.
Published 6 years after the original trilogy, it was supposed to be a one-off as there was no idea if a market for more original Arduin material existed. However, it sold well enough that Dave went ahead with four more volumes after this.
Comments on A&E #505; A Primer on Arduous Adventures in the Kingdom of Arduin: An Open-Table Advanced Dungeons
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