"It's complicated." And certainly wasn't natural causes. Strange pauses for a moment, thinking of how best to explain this without going off into a big nerdy rant. He's going to tell Rita the truth, of course. But phrasing it as 'we summoned the Raven King but then tried to give him all of English magic symbolically as a gift but instead shoved it in a butler who then proceeded to use said English magic to murder a faerie' raises way too many questions.

"Someone was used as a vessel for all of English magic. Once that happened, he was able to strike against the fae with the power and entirety of magic itself. Only that was enough to defeat the fiend once and for all."