The upcoming appearance in Action Comics #894, scheduled for release on October 27, of the Sandman's more popular older sister Death has been touted as the first appearance of the character in a mainstream DC Universe book. Those making this claim seem to have conveniently forgotten 1990's Captain Atom #42. In that issue the Captain journeyed into the afterlife and met up with several death themed characters, including the Black Racer and Nekron, as well as Death herself. The story appears to be an ill-fated, ill-advised and wholly unnecessary attempt by Captain Atom writers Cary Bates and Greg Weisman to further streamline post-Crisis On Infinite Earths DC continuity by reconciling all the various incarnations of the Grim Reaper who had appeared in the company's magazines over the years.
The main effect of story was to piss off Neil Gaiman. He complained that Death acted horribly out of character in the story and insisted that Death of the Endless was not an aspect of Death, as Bates and Weisman suggested, she IS Death. Period. So incensed was Gaiman by this perceived mishandling of the character that he laid down the law to DC's editors, decreeing that Death, or any other character from Sandman, could not be used by any other writer without his say so.
It was a pretty ballsy move on Gaiman's part to presume to dictate to a major corporation just how they could utilize their intellectual property. Nonetheless, it worked. DC caved, and Death, though certainly not death, was banished from the mainstream DC universe. It perhaps helped Gaiman's case that Sandman was one of the best selling and most critically acclaimed series that DC has ever published. Otherwise, they might just have shown Gaiman the door and turned the book over to Paul Kupperberg.
It might have worked too well. There is a story that the writers and editors of the Superman line would have liked to have included a cameo by Death in the "Death of Superman" storyline, and, furthermore, that Gaiman might even have been disposed to look favorably upon such an appearance. Yet, seemingly so wary were they of invoking his ire that they didn't even ask.
Current Action Comics writer Paul Cornell assures us that Death's appearance in his story has been fully sanctioned by the Great and Terrible Gaiman.