Everybody's got secrets. Some are told, and become something less than secret. Some, of course, are never told. They are taken to the grave, held tight in human hearts until those hearts stop beating. And then there are the secrets that really matter: the secrets so strange, so terrible, that the people who hold them just disappear, falling out of reality in the infinitesimal space between one second and the next.
Born Under an Ill Omen
Ambrose Bierce was the tenth of thirteen children, part of a family remarkable only for its size. Large families like Bierce's were perhaps less remarkable in 1842 Ohio, but there's ample numerological significance surrounding the number thirteen. Bierce was born in late June, suspiciously close to the Summer Solstice (when, some believe, the year begins to die). It's no blood-red comet in the heavens, and certainly no Star in the East, but it is, perhaps, enough to herald the birth of a weirdness magnet like Ambrose.
Little is known about Bierce's upbringing and childhood, though it's probably safe to say -- despite that pesky thirteen -- that he wasn't reared as part of a witch's coven by parents he described most frequently only as "literary." Of course, if you're planning to use Bierce in your Secret (or Alternate) History game, there's little enough on record that it's difficult to prove he wasn't part of a family of powerful sorcerers and occultists.
That said, if Bierce's family was partaking in that old black magic, we'd be looking at a heady bit of irony. Bierce could trace his ancestry back to Puritan pilgrim William Bradford, one of the stalwart founders of the Plymouth colony: a genuine founding father. Though Bradford was decades removed from the whole Salem fiasco, it might make more sense to make Bierce's family of thirteen witch hunters, carrying on a grand family tradition stretching back to the Mayflower.
A Dangerous Warrior
No matter which side he was on, there's little doubting that Bierce was a warrior. He served as an officer in the U.S. Army during the Civil War, fighting with distinction at Phillipi, Rich Mountain, Shiloh, and eventually Kennesaw Mountain, where he sustained a major head-wound that put a temporary halt on his military career. He returned to the army after the war ended, joining an expedition across the still only partly-explored Great Plains, starting in Omaha, Nebraska and ending in San Francisco (the home of America's first and only Emperor, who was one of Bierce's contemporaries).
If Bierce's childhood doesn't explain the strangeness that would eventually surround him, his experiences in the War Between the States just might. Our witch-hunting Bierce might have been a sort of prototype for Manly Wade Wellman's devil-battling Silver John, clearing the misty Appalachia of old cults and strange creatures who ruled the earth before the rise of man. There's certainly mundane horror aplenty for our Bierce to have experienced in battle, but even a few brushes with the supernatural (taking inspiration perhaps, from the well-conceived but less well-executed Abraham Lincoln: Vampire Hunter) might explain his later fascination with death, high weirdness, and lost cities. Or maybe that blow to the head opened Bierce's mind to things average men cannot perceive. Regardless, after the war, Ambrose became sullen and withdrawn. Even his friends began to call him "Bitter Bierce."
Go West, Young Man
After arriving in Norton's Imperial California, Bierce left the Army to become a freelance journalist, carrying on his families maddeningly non-specific "literary" heritage. Your call if he was authoring (or burning) grimoires in his spare time. In real life, he was renowned as a crime reporter, maybe as cover for his own unspeakable crimes (or his righteous battles against those who committed them). We're in an increasingly Eastern-influenced San Francisco, after all: maybe this Bierce is the 19th Century inspiration for Jack Burton in Big Trouble in Little China. From Appalachian hoodoo to Oriental mysticism in no time, flat.
Although Bierce spent most of his time in San Francisco, he spent time in London (if you need inspiration here, try looking at Pelgrane Press's fantastic Occult Guide to London), and then the Black Hills of then-Dakota Territory (now South Dakota), where he briefly managed a mining company operating between Deadwood and Rockerville, hobnobbing with some of the legendary figures of the Old West. I'm sure that's enough to make your Bierce a treasure hunter or a gunslinger (or both), so I won't even bother to remind you that the Black Hills are shot through with ancient caves and sasquatch sightings, or that they're considered dangerously sacred to the Lakota Sioux. Maybe a gateway to the spirit world? Maybe just the foothills to Lovecraft's Plateau of Leng. Or maybe the first steps on a long trip to the shores of Lake Hali, beneath Aldeberan and the Hyades.
That's right: we're not done with Bierce quite yet. We've got more to say about the Man from Carcosa.