Thammuz is a minor demon from the eighth dimension.
Thammuz was summoned by Magnus Bane in 1878 during his search for the blue demon that apparently cursed Will Herondale. Upon being brought before Will, Thammuz immediately began taunting and threatening Will with violent acts, for which Magnus chastised him by making the candles forming his summoning circle blaze up. When Will told Magnus of his doubts that Thammuz was not the correct demon, Will called him a "periwinkle" blue, enraging the demon, leading Thammuz to intimidate Will further. Will then became assured that Thammuz was the wrong demon because he sounded different and had an extra eye.
Wanting to be sure, Magnus compelled Thammuz into telling the truth, through a chant from his book, about whether or not he has ever encountered Will or his relatives in the past. Thammuz initially replied that all humans look alike to him, but eventually admitted that he believes he had never seen Will before, remarking that Will looks like he would have been tasty and memorable, and that the last time he was in their world, the humans were still "living in mud huts and eating bugs".
Will then asked him about the blue demon he had just described. Thammuz told him that there were hundreds upon hundreds of millions of demons in the Void and Pandemonium. Determining that he was useless, Magnus banished him.
Thammuz is no more than three feet high. He has pale blue skin, three coal-black, burning eyes, and long, blood-red talons on its eight-fingered hands.
- Clockwork Prince (first and only appearance)