Brutal tactic finishes off strong opponent, (vs 1885)

In case you’d like to see the entire game, you’re welcome to check it out here: An even fight leads to a rook and and minor piece endgame, where I am able to make steady progress before my opponent blunders. Then a move that looks ridiculous at first sight puts the game away.

This position looks like a number of moves would win – eventually, but here there is a move that means death on the spot. Can you find it?
Ra8! Cutting the king and rook off from the last rank to promote the pawn, and if the rook is taken – as it was in the game – pawn c8=Q, checkmate!