I was just reading about the Star Trek: Legacy video game that was co-written by DC Fontana. Apparently one of the things it deals with is the Vulcan/ Romulan split which is referred to as the Sundering.
I am curious where this term originated from. Does it come from the TV shows because I don't remember it. Does it come from DC Fontana's other works? Does it come from Diane Duane's Rihannsu stories or does it originate somewhere else? If it does come from Diane Duane's works, does this mean Duane's and Fontana's works coincide on Vulcan/ Romulan history?
It's not from the shows. I believe Duane originated it, though it may have come from fan fiction. And no, using the same word doesn't necessarily mean the continuity is the same; it's just an homage. "Sundering" seems to be a fairly common word in fantasy fiction; in addition to the Warhammer reference mentioned above, there's a Jacqueline Carey fantasy duology called The Sundering
(and an SF novel of the same name by Walter Jon Williams). Like most things in high fantasy, it originates in Tolkien, which contains references to "the Sundering of the North" and "the Sundering Seas." "Sunder" is simply an old-fashioned English word meaning to split, separate, or divide (as in the Biblical "let not man put asunder" -- it's also related to "sundry," meaning various or diverse).
Duane has written a lot of fantasy, so she probably introduced the term "Sundering" into Trek, and others followed her lead.