From Discworld & Terry Pratchett Wiki
The Summoning Dark is a pan-dimensional creature and spirit of vengeance from dwarf mythology. The Dark's mine sign rune is described as like "a floating eyeball with a curly tail". The Summoning dark is one of various "Darks" that occur in dwarfish mines - among these are the Following Dark, the Calling Dark, and the Long Dark. The Summoning Dark is described as the worst of the Darks, something a dwarf would call upon only in the most desperate of circumstances. Traditionally, light keeps the Dark at bay - for example, in Thud!, the dwarfs of the Ankh-Morpork City Watch place candles around a version of the sign in order to contain it.
Mustrum Ridcully told the Patrician that he believed the summoning Dark to be be a quasi-demonic entity that was untold millions of years old. It came into being at the beginning of the multiverse, but as it drifted near no major accretion of belief (such as the gods), it tumbled through the void, semi-sentient and bereft of purpose.
The Summoning Dark is an entity that has existed for uncounted years, but has not always embodied a Dark. According to Thud!, before becoming part of dwarfish lore it "found work as a superstition".
Appearance in Discworld novels
The Summoning Dark is first mentioned in Thud!, where it plays a major role. It is called into Ankh-Morpork by a dwarf who is stabbed, locked up and left to die beneath the city by the Dark Guards, most likely by order of Grag Hamcrusher or Ardent. The Dark was scrawled on the door that had been locked by the Dark Guards in the blood of the dying dwarf. This inscription is found by Angua and Sally von Humpeding of the Watch in their investigation into the murder of Hamcrusher. The Summoning Dark possesses Samuel Vimes after he pricks his thumb on a "nail" in one of the large metal doors in the Ankh-Morpork Dwarf Mine. The Dark gains more and more control over Vimes over the course of the novel, but is finally beaten, as Vimes creates his own Dark - the Guarding Dark, as he terms it. The Guarding Dark was created by Vimes as a means of controlling his potential "dark side" (see the main Samuel Vimes article).
The Summoning Dark returns in Snuff and it is implied now that Samuel Vimes and the Dark share a mutual respect; Vimes often meeting it in dreams where they nod to each other and go their separate ways again. Vimes also carries a scar left by the Dark on his arm which seems to still hold some supernatural power that gives him a vision of the goblins' demands for 'just ice' after the goblin Stinky touches it. The Dark has also given Vimes perfect night vision, as he enters the pitch-black goblin cave, Vimes realizes he can see perfectly. Vimes also seems to be able to communicate with the Summoning Dark, the condition being he needs to surround himself in darkness: with this skill he is able to acquire a witness account from the Summoning Dark, as the Dark represents all darkness: "This is wrong, Vimes told himself. How can you take testimony from a demon, especially when it's one of no fixed abode?" With the Summoning Dark by his side Vimes could easily solve all his cases, however he is reluctant and cautious to use the entity as he realizes that the more he relies on the Dark the more vulnerable he becomes to succumbing to his own 'dark side'.
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