Described in The Colour of Magic. The Arch-Astronomer of Krull was the despotic ruler of the the small and secretive kingdom of Krull, whose rim-most mountains project out over the Rimfall. He ruled in a particularly violent fashion, having those who displeased him sentenced to excruciating deaths, but he did preside over a populace of intelligent and searching individuals. Variously described as cosmochelonians and astrozoologists, many of the Krullians were deeply interested in Great A'Tuin and - specifically - the sex of the great Star Turtle.
Amongst the responsibilities that go with the post of Arch-Astronomer is the occasional regrettable necessity to put to inconvenience craftsmen such as Goldeneyes Silverhand Dactylos lest they shame Krull by going on to do bigger and better things for other people, and in this specific case possibly being eaten by the Luggage.