Yeah, "Magnus" is just Latin for "Great." So he's Gaius (family lineage) Tavarus (new name he chose for himself; many, many kings picked a new name for themselves in history when they took the throne—King George VI, the latest King of the United Kingdoms (who died in 1952) was born with the first name "Albert," and only chose the name "George" after his older brother abdicated. "The King's Speech" actually did a really good job in terms of reasonable accuracy about the abdication and King George's succession. Spoiler alert, I guess. The point is that normally a king or ruler would select a name from a previous ruler, unless they wanted to break from tradition for whatever reason; Tavi's succession to First Lord fits the mold) The Great. It helps that Tavi never really went by Octavian; he'd been Tavi all his life, and his favorite enemy gave him a name of his own. It just happened to fit his current nickname.
