The Raw where this mystery person appears is the one after Elimination Chamber. I think it will be the Undertaker, here's how it will be booked.
At EC Edge retains. Barrett is eliminated controversally
On Raw the following night Barrett demands a No DQ title match against Edge for the title as he was wrong, it is granted.
During the match, Barrett sets up for the Wasteland. Lights go out, gong, Taker appears and tombstones Barrett. Lights out, gong, Taker disappears. Edge spears Barrett for the win.