An early-career Gaiman co-wrote the novel Good Omens (subtitled "The Nice and Accurate Prophecies of Agnes Nutter, Witch") with Terry Pratchett in 1990. He had previously vetoed any chance of a screen version in the wake of Pratchett's death, saying that they had a pact to only work on Good Omens projects together. But a letter from Pratchett to Gaiman, written to be read posthumously, gave Gaiman the necessary blessing to go it alone.
Combining the best of both Pratchett and Gaiman's writing styles, Good Omens is by turns dark, philosophical and barkingly funny. Angel Aziraphale and demon Crowley have got rather fond of Earth and are thusly annoyed to discover that the Antichrist is going to show up and bring it to an end. Terry Gilliam was once planning a film adaptation with Johnny Depp and Robin Williams, but was scuppered by 9/11. Hollywood wasn't up for a "hilarious movie about the anti-Christ and the end of the world" at that point, Gaiman told us in 2013.
Gaiman hasn't said who he's writing the TV series for, but at six episodes you might assume the BBC. He also wrote an adaption for BBC Radio 4 in 2014, in which Pratchett had a cameo.