Len Wein, the best-selling comic book writer and editor who co-created Wolverine and Swamp Thing and edited Watchmen, has passed away. He was 69. The comics community is beginning to respond on social media after Spider-Man writer Brian Michael Bendis tweeted the sad news out. In recent months, Wein had undergone a number of surgeries, with the final tweet on his official account indicating that he came through surgery, “which went very well,” on September 7.

Wein’s first professional comics story was “Eye of the Beholder” in 1968’s Teen Titans #18, which he co-wrote with soon-to-be-Titans legend Marv Wolfman. He is best known for two original creations of his that have forever altered the landscape of superhero comics: Wolverine and Swamp Thing.

Wein’s Swamp Thing co-creator Bernie Wrightson passed away earlier this year.

In the 70s, Wein wrote regularly for Marvel Comics, beginning with a single-issue story in Daredevil #71 co-written with writer/editor Roy Thomas. He later succeeded Roy Thomas as editor-in-chief of Marvel’s comics line in 1974.

Wein and artist Dave Cockrum revived the X-Men in 1975, creating an army of new characters to populate the book, including Nightcrawler, Storm, Colossus, and Thunderbird; this is when Wolverine, created earlier by Wein with artists John Romita Sr. and Herb Trimpe, joined the team. Wein and Cockrum plotted out the next few issues, essentially priming the pump for writer Chris Claremont, who scripted the issues and began a legendary run on the title.

After Wein’s relationship with Marvel management soured in the late ’70s, Wein went to DC as a writer and eventually editor, where he first collaborated with Watchmen’s Dave Gibbons. He also worked with John Ostrander to co-write the Legends event miniseries, where the modern-era Suicide Squad was introduced.

Besides Watchmen, Wein worked on The New Teen Titans, Batman and the Outsiders, Crisis on Infinite Earths, and All-Star Squadron as an editor. Following the events of Crisis on Infinite Earths, he wrote the Ted Kord Blue Beetle series, as well as providing scripts for the start of George Perez’s groundbreaking reinvention of Wonder Woman.

After leaving DC, Wein was editor-in-chief of Disney Comics for three years in the early 1990s. He next headed to TV, serving as a writer and story editor on X-Men, Batman, Spider-Man, Street Fighter, and more.

In the latter part of his comics career, Wein and Wolfman wrote a one-shot titled Gene Pool for IDW, based on an unproduced screenplay the two had developed, and so much more, with more recently a return to Swamp Thing.