Slade is alive.
The rest of the team think that Robin is hallucinating because only he is seeing Slade. Later they find that Robin has been kidnapped by Slade when he reverses that he has indeed been there the entire time. He has mastered hiding himself so that only Robin would be the one that sees him. They now have a limited amount of time to save him. Filled with guilt them gang teams together with the members of the the other Titans to rescue and save Robin.