Complete solution including diagrams (congrats Hive account@udow for spotting it):

1. ... Rb8! Idea: 2. ... Qb6# checkmate.

White has now several options. I will consider:

a)

2. Kxb8 Qb6+
3. Kc8 Ke7# Checkmate. A kings move finishes the game by giving discovered check.

b)

2. Qg1 Ke7! Threatens 3. ... Rhc8#.
3. Kc6 Rhc8+
4. Kd5 Rc7! Threatens Rd8#.
5. h8Q Rd7+
6. Kc6 Rb6# Checkmate.

c)

2. Ra6 Ke7!
3. Kc6 Rhc8+
4. Kd5 Rd8+
5. Kc6 Rb6+!
6. Rxb6 (The variation 6. Kc7 Rd7+ 7. Kc8 Rxa6 8. Kb8 Ra8+ 9. Kxa8 Qa7# is one move longer.) 6. ... Rd6+
7. Kxb5 Qxb6+
8.Kc4 Qc6# Checkmate.

Maybe it's interesting for you to know that a recent blitz chess game which I won with the black pieces inspired me to to create this chess problem. The game was full of inaccuracies but nevertheless already contained the key idea of this riddle:

1. e4 c5 2. d4 cxd4 3. Nf3 a6 4. Nxd4 Nf6 5. Nc3 e5 6. Nf5 d5 7. f3 Bxf5 8. exf5 Be7 9. Bg5 d4 10. Ne4 Nxe4 11. Bxe7 Qxe7 12. fxe4 Qh4+ 13. Kd2 Qf4+ 14. Kd3 Qe3+ 15. Kc4 b5+ 16. Kb4 Nc6+ 17. Kc5 d3+ 18. Kxc6 Rc8+ 19. Kb7 Rb8+ 20. Kxb8 Qb6+ 21. Kc8 Ke7# Checkmate.