Nightmare's theme is titled "Pavor Nocturnus", another name for the sleep disorder "Night Terror".
This appears to be a reference to Spoiler:the secret boss character, Night Terror, from Soul Calibur III.

Mitsurugi, Hilde, and Siegfried are the only returning characters that show physical signs of aging since the previous installments.

Originally, the game's director, Daishi Odashima, wanted to name the game "Soul Edge 2," possibly as a way to introduce new fans to the first game in the series. However, Namco refused it.

Soul Calibur V's story mode was originally planned to be four times bigger than it actually is, but time restraints meant it needed to be cut back.

Soul Calibur V is the first game in the series to do away with individual endings in the arcade mode, which means several playable characters amount to simply cameo appearances in the single Patroklos-focused story mode, and quite a few make absolutely no appearances at all in the story.

Katsuhiro Harada, producer of the Soul Calibur series and the Tekken series appears as a bonus character in a Quick Battle challenge, using the Devil Jin style moveset. Defeating him will unlock the achievement "Pursuit of Obsession".
His theme music is from the Cathedral stage of Tekken 5, his appearance is very similar to Heihachi from Tekken, and his profile image shows him without sleeves, even though in-game he wears a longsleeved gi.
His name appears in game as "Harada_TEKKEN" - the same as his Twitter handle.