Historical Fantasy, more accurately described as Historical Fiction, is a genre/form of Setting Aesthetics which revolves around telling stories in the historical past of Earth, exploring things that might have happened but didn't actually happen in our reality. Despite its name, Historical Fantasy does not necessarily involve the presence of magic or fantastical beings; if it does, it's usually only to the extent of Low Fantasy, although Urban Fantasy in a prehistoric setting is not unheard of. This genre is more likely to revolve around ideas such as "What if the Roman Empire didn't fall? What if Julius Caesar was never assassinated? What if Hitler won World War 2?"
HF and D&D
Despite what you might think, Historical Fantasy and Dungeons & Dragons have actually crossed paths quite a lot. In Advanced Dungeons & Dragons, TSR released an entire series of splatbooks - the HR series - dedicated to exploring this angle of gaming, due to its close relationship to the Sword & Sorcery fiction that Gygax had loved and tried to build D&D to emulate. Wizards of the Coast would follow up with the D20 Past splatbook for D20 Modern, which is literally a Historical Fantasy basic corebook.
The HR series consisted of the following books:
- HR1 - Vikings Campaign Sourcebook
- HR2 - Charlemagne's Paladins Campaign Sourcebook
- HR3 - Celts Campaign Sourcebook
- HR4 - A Mighty Fortress Campaign Sourcebook
- HR5 - The Glory of Rome Campaign Sourcebook
- HR6 - Age of Heroes Campaign Sourcebook
- HR7 - The Crusades Campaign Sourcebook
Dragon Magazine would also get in on the act; Issue #257 would feature the article "Dark Ages: Roleplaying in Anglo-Saxon Britain", which examines the timeline, cultures, and geography, as well as providing new kits and several new demihuman races based on Anglo-Saxon myths - the Fey, Alfar, Dockalfar, Erlking and Trow. This would be followed up in issue #263's "Hearth & Sword: Deities of the Dark Ages", which provides Deities & Demigods style writeups for the Anglo-Saxon pagan pantheon, as well as Christianity.