The comic book world lost one of its most talented artists this September as John Cassaday has passed away at the age of 52. Cassaday, who broke into the business in the 1990s, was an Eisner Award-winning creator behind so many acclaimed runs of much-loved characters and properties, most notably Marvel's Astonishing X-Men (2004), as written by The Avengers director Joss Whedon.

In addition, Cassaday worked on John Ney Rieber's Captain America run (2002) and Jason Aaron's Star Wars comics (2015). Fans of independent comic publisher Image Comics will also know of his work on Planetary, the original creator-owned sci-fi comic from writer Warren Ellis. Outside of the comic book world, Cassaday worked as a concept artist on films such as Daredevil (2003) and Watchmen (2009), as well as directing an episode of TV show Dollhouse (2009).

Cassaday never lost his passion for his work and was developing new projects at the time of his unexpected death.

Cassaday died on Sep. 9, but it was a few days earlier that the internet was alerted to his ill health. On Sep. 4, his sister, Robin K. Cassaday, announced on her Facebook account that her brother had been admitted to hospital due to an unknown serious condition.

Robin noted in her initial post that "he is still stable and his organs and heart are working, but his brain is the main concern. He is supposed to have an MRI this Saturday, which will give an even clearer picture and pinpoint exactly what is going on."

Tragically, it seems Cassaday's condition only worsened as by the 9th, Robin had a heartbreaking update to share with the artist's fans: "My little brother John passed away at 1:51 am this Tuesday. Please say a prayer for me and mom in your prayers."

At the time of writing, then, John Cassaday's exact cause of death has yet to be confirmed, but it's clear that it was the result of a sudden and short illness. His passing has been mourned by many of his friends and contemporaries in the comic book industry, including writer Mark Waid (Kingdom Come). "My dear friend John Cassaday passed today at the absurdly young age of 52 and I miss him already," Waid wrote on social media, in the wake of the sad news.

"John Cassaday, I will say without hesitation and with very little fear of disagreement, was one of the very best illustrators and storytellers to ever work in the comics medium," Waid concluded his moving post. "Most people are lucky if more than a dozen people are still talking about them a month after they pass. My friend John will be talked about and remembered by an entire industry for ages. And rightfully so. Rest in peace, sir."

Cassaday is survived by his sister, his mother, and his partner, Tara, as well as the millions who have been awed and inspired by his art over the past three decades.