This is a subset of Historical Fantasy and so shares the defining feature of an historical setting. The difference is that it also shares aspects of High Fantasy—detailed epic tales of heroism.
Expect a high degree of detail—lots of research went into the crafting of these stories and the authors will prove it to readers. Indeed, authors are able to craft fictional and fantastical stories that seem almost possible. High Fantasy has strong world-building, but when writing a High Fantasy in an historical setting, writers build on a preexisting structure. The world is still well developed, complicated, and vast, but newly familiar.
High. Magic fills the world of High Fantasy and so too does it permeate the pages of a High Historical Fantasy.
Variable. This feature depends entirely on authors' intentions. There is usually a reason authors pick a specific time period/figure/event. When returning to history there is something to be said.
High. The heroes of these stories are larger than life, they are capable of great feats. Writers draw on preexisting biographies of historical and legendary figures in order to create complex characters.
High. In High Historical Fantasy the sequence of events and the relationships between cause and effect are incredibly important because these are the pivot points of history and dictate how the plot will unfold. Expect intricate plots that span entire ages with pieces slowly falling into place.
High. The stakes are high in this sub-genre with the fates of nations and cultures hanging in the balance and violence is a natural component to survival. However, these stories are bigger than swashbuckling adventures, so while violence may be key to the story's events, it is not usually the narrative focus.
Historical Fantasy and High Fantasy. This sub-genre is a combination of these.
Celtic and Arthurian Fantasy. Both of these sub-genres take place in about the same time period, and while they may lean more to the fantastical than historical, nonetheless can be considered a part of the sub-genre.
Wuxia. Not all High Historical Fantasies are set in a world resembling European history, sometimes, as in Wuxia, it is an Asian setting complete with its own history and mythology.
By Poul Anderson. A retelling of the legendary 6th century Danish king Hrolf Kraki. Anderson draws from a diverse array of source material, including mythological poems.
By Hiroshi Aramata. (The Tale of the Imperial Captial). An epic story spanning 20th century Tokyo from an occultist perspective.
By Stephen Lawhead. Based on Arthurian legend and only partly historical the series presents a story with a sense of reality.
By Guy Gavriel Kay. Clashing conquerors, sorcerers, concubines, schemes, vengeance, destruction, romance, freedom—this epic story about a land that resembles medieval Italy is fraught with suspense.
By Susan Cooper. The setting is our world, but rooted in a mythological Celtic past.
By Jack Whyte. Spanning generations these stories are about the survival of a culture as the Roman Empire collapses.
By Albert A. Dalia. The Adventures of the Shaolin Blad Tanzong. Mythological and historical, this novel is about holding together a newly established dynasty.
By W. Michael Gear and Kathleen O'Neal Gear. It's 1300 AD and great armies are marching. In what is now Alabama and Mississippi The novel weaves the quests of three wanders who wish to create peace.
By Nalo Hopkinson. An atypical example, this book is epic in its scope traveling across centuries and countries. It is about hardship and struggle. It is a book about historical narratives and identity. It touches on the biblical and the mythic
By Marion Zimmer Bradley. This series retells the Matter of Britain, specifically in regards to King Arthur. The stories span centuries and are told from the perspective of powerful women.