A.T. Robertson Commentary Revelation 17:7

"And the angel said unto me, Wherefore didst thou wonder? I will tell thee the mystery of the woman, and of the beast that carrieth her, which hath the seven heads and the ten horns." — Revelation 17:7 (ASV)

I will tell thee the mystery (εγω ερω σο το μυστηριον). The angel gives his interpretation of the woman and the beast (17:7-18). Ερω is the future active of ειπον (defective verb), to tell, to say.