Mirek, as a representative or proxy, set up a meeting with Sebastian in Prague. When they met at a plaza near Charles Bridge, Mirek became suspicious of Sebastian for bringing along Clary and Jace, as the agreement was he would only deal with him. He was distrustful of the two so Sebastian told him that they were all siblings; because Mirek knew that Valentine had one son and a daughter, Sebastian said Jace was adopted.
Finally, Sebastian brought out a pouch of bronze coins, and Mirek, after considering it, decided that it was not enough for the pure adamas that Sebastian wanted. This infuriated Sebastian as they had agreed on the price, and Mirek retorted that Sebastian also agreed to come alone. Mirek then asked for a lock of Clary's hair, which Clary almost gave, though both Jace and Sebastian objected because Mirek was a dark magician and could have done unimaginable things with her hair.
Mirek then taunted Sebastian about being unlike his father who commanded respect and loyalty. Sebastian retorted that he was more powerful than Valentine, and that he will have them through their fear, threatening to kill him and take the adamas. Sebastian then activated his seraph blade "Dumah", scaring Mirek, not knowing that it was because a swarm of Elapids had appeared around them.
During the skirmish, Mirek tried to escape but was caught by Clary. Mirek tried to fend her off and even offered her "whatever" she wanted, though Clary, still under the influence of her momentary adrenaline, declined and killed him with her seraph blade "Nakir." The adamas rolled under the counter where he died and disappeared, and Clary retrieved it for Sebastian.
Mirek looked like a typical Vetis demon with his tall humanoid with gray skin, ruby-red eyes, long, serpentine arms that ended in heads like an eel's, and a mouth full of pointed teeth that jutted in every direction.
While glamoured during his meeting with Sebastian, Mirek took the the form of an old man with a wrinkled face. Clary mistook him for a mannequin.