In September 2014, Gilbert Cosme began working for Lucha Underground under a mask and the ring name Mil Muertes. In storyline, Muertes' real name was Pasqual Mendoza, whose entire family died in an earthquake in Mexico City, when he was seven years old. He debuted on the episode, which aired on November 6, 2014, accompanied by Catrina as his valet. He was then involved in a major rivalry with Fénix (punctuated by Catrina's turning on him and aligning herself with Fénix), which concluded when Fénix defeated him in a "Grave Consequences" casket match that aired on March 18, 2015. After being "resurrected" by Catrina, Muertes returned and on April 19, 2015, at Ultima Lucha, Lucha Underground's season one finale, defeated Prince Puma to win the Lucha Underground Championship.
During the season two premiere on January 27, 2016, Muertes successfully defended the Lucha Underground Championship against Ivelisse.