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"Heart of Darkness" finds Elena and Damon traveling to Colorado to meet up with Jeremy-in-exile. Why? Apparently they want to use Jeremy's intermittent talking-to-the-dead talents to ask the vampire Rose who sired her. That way, the Salvatores will be able to figure out which Original needs to stay alive.
Because it wouldn't be any fun to just watch Jeremy talk to empty air, Rose herself (Lauren Cohan), does indeed make an appearance.