from another site yesterday.  White = g3 h3 Rb4 Rd4 Qb8 Kg4  Black = g5 h6 Qa5 Kf6

My last move was B8(Q)   Resignation followed immediately.  A lesson for us all

From where did the Queen move?

I resigned before black could play Qf5+ then Kh5 then g4+ wins

I think unique promoted on their last move.

Yes, that makes sense. If we take back that move, then Rd6+ is mate in 8 moves according to Stockfish. :)