C'mon, think about it. If you were Cain, host of the House of Mystery, wouldn't you want someone named 'Gravedigger' working for you, too? Makes a whole lot more sense than hiring Aqualad, for instance.
Then again, we've seen an incredible range of team-ups over DC's history, so perhaps I'm being a little hasty in throwing out that HoM/Aqualad possibility. In fact, team-ups have proven so popular that entire series have been devoted to them, including DC Comics Presents (starring Superman), The Brave and The Bold (mostly starring Batman), and Super-Team Family (which died a merciful death, thanks to the great DC Implosion of the late 1970s).
The first DC team-ups came out of the Golden Age, with several heroes coming together as the Justice Society of America to fight common foes (although the format was for each hero to fight in individual battle, and then all join together for a single final chapter). Over time, this gave way to full-fledged interactive team-ups, much to the delight of Roy Thomas, Jerry Bails, and lots of other tykes.
Meanwhile, Superman and Batman held forth in World's Finest Comics, spending way too much time dressing up in each other's clothes, ostensibly to fool their respective lady loves into not figuring out anyone's real identity. Forget about all of the Batman and Robin rumors; when two heroes keep coming out of the closet wearing each other's spandex, all you gossip-mongers out there have bigger fish to fry.
When super-heroes re-emerged from their mid-fifties hiatus, even in their new incarnations it didn't take them long to begin hooking up. The Justice League of America was clearly a rebirth of the old Justice Society book, and books like The Brave and The Bold and Showcase kept the team-up traditions going.
DC published some very nice stories in the Showcase title, experimenting with a few unusual pairings. One Flash/Atom story with art by the incomparable Alex Toth particularly stands out (it's included in the Greatest Team-Up Stories Ever Told trade paperback, incidentally). Eventually, though, sales figures paved the way for Batman to take up permanent residence in the book, with a new guest star in each issue. (Such a prima donna; you'd think he'd have been happy with two books, but, no, he had to take over a third one, too...)
Other winners in the DC team-up sweepstakes would have to include Batman and the House of Mystery, Superman and Santa Claus, and - of course - one of my personal favourites, Angel and The Ape and The Inferior Five. You da man, Phil.
Getting back on point, Gravedigger made his debut in the first issue of Men Of War, published in 1977. A polio victim as a child, he had pushed himself sufficiently to overcome his physical impairments, and later enlisted in the army to fight in the Second World War. Due to his skin colour, he was assigned to permanent grave-digging duty - hence the nickname - rather than to any front-line combat unit.
Upon hearing that a friend had been killed, he went AWOL and somehow made it back Stateside, where he invaded the offices of the Joint Chiefs of Staff, inside the Pentagon in Washington, D.C., in order to prove his stealth and combat abilities.
Apparently, he passed the audition, since instead of throwing him in Leavenworth, as might have been expected, the Joint Chiefs designated him as a one-man commando unit, and aimed him at the enemy. They probably figured that if they didn't, he'd just escape and come after them again, so their decision was reasonably prudent. According to DC's Who's Who, it's undetermined whether he survived WWII, so here's a chance for someone to come up with a new revamp. Any takers from among the I-wanna-be-a-comic-book-writer contingent?