Answer: Re8! Main line : 1...Re8! 2.Rxe8 Bf5+ 3.Re4 Bxe4+ 4.Q | Chessgame

Answer: Re8!

Main line : 1...Re8! 2.Rxe8 Bf5+ 3.Re4 Bxe4+ 4.Qc2 Qxc2#

The main idea is to open up the b1-h7 diagonal for the black bishop.
However, at first glance the black rook on e4 is blocking the b1-h7 diagonal for the bishop.
In addition, black has to create a threat on the next move since the black queen on d2 in under fire.
Re8! accomplishes just that, as it pins the white queen to the king and forces the capture with the rook on e1.