Charles Ellicott Commentary Genesis 20:3

"But God came to Abimelech in a dream of the night, and said to him, Behold, thou art but a dead man, because of the woman whom thou hast taken. For she is a man`s wife." — Genesis 20:3 (ASV)

God (Elohim) came ... — From the use of this title of the Deity, it has been said that this narrative is an Elohistic form of the Jehovistic narrative in Genesis 12:10-20. But we have seen that even in the History of the Fall, where the writer in so remarkable a manner styles the Deity Jehovah-Elohim, he nevertheless restricts Eve and the serpent in their conversation to the name Elohim.

With the same care in the application of the names, it is necessarily Elohim who appears to a heathen king; and if the title Jehovah had been used, it would have been a violation of the narrator’s rule. Moreover, the sole reason for calling that narrative Jehovistic is that in Genesis 12:17 it is Jehovah who plagues Pharaoh for Sarah’s sake. But equally here, in Genesis 20:18, it is Jehovah who protects Sarah from Abimelech; in both cases it is the covenant God who saves His people from injury.

You are but a dead man. —Hebrew, you die, or are dying. Abimelech was already suffering from the malady spoken of in Genesis 20:17 when Elohim appeared to him and warned him that death would be the result of perseverance in retaining Sarah. It was this malady which was the cause of the abstention spoken of in Genesis 20:4; Genesis 20:6.