Sad tributes are pouring in for the Marvel artist who worked on the X-Men, who died at the age of 52.

Marvel comic artist John Cassaday died on Monday at the age of 52. He was best known for his work on Astonishing X-Men.

Many moving tributes have been written for the artist, who was remembered by fans as a truly unique artist.

Robin, Cassaday’s sister, was the first person to tell people that her brother had died.

There was a death in my family at 6:51 p.m. Texas time. My sister told him, “Please pray for me and my mom.”

They haven’t said what killed Cassaday yet, but Robin did say that he was in the intensive care unit for six days before he died.

Robin said he was stable at the time, but he was worried about his brain.

“All of his organs and heart are working normally. “My main worry is about his brain,” Robin wrote as she asked for prayers.

Before working for Marvel and DC, Cassaday got his start in the comic book business in 1997, when he started working for Dark Horse and Homage Comics.

While working for two of the biggest comic book companies, Cassaday wrote stories for the X-Men, Superman/Batman, and Captain America.