Marbas was a known cunning creature who has fooled many powerful magicians. Eventually, he was trapped in a Pyxis by Edmund Herondale.
Marbas spent nearly twenty years in the Pyxis until Edmund's son, Will, accidentally released him. Marbas expressed his desire to kill Edmund, but since he was not there, he settled for the children in front of him. Will's sister Ella fought him, and Marbas stung her with his poison, killing her hours later. Before he left, he told Will that he had cursed him so that everyone who loves him would die.
Years later, Marbas went to Benedict Lightwood's ball and was attacked by Will. He wounded Will and escaped, leaving behind a tooth in Will's wound. With this, Magnus Bane summoned him and asked him to remove Will's curse. Marbas then revealed that he was in fact unable to cast the curse, as he had been at his weakest. Magnus banished him after.
Marbas is a short demon with reptilian features, blue scales, scarlet eyes, a flat snakelike snout, and a long, yellowish barbed tail with a stinger on the end.