Genesis 20:3
“But God came to Abimelech in a dream of the night, and said to him, “Behold, you are a dead man, because of the woman whom you have taken. For she is a man’s wife.””
What does Genesis 20:3 mean?
God intervenes directly here to protect Sarah and warn Abimelech that he's unknowingly about to commit a serious wrong. Abraham had told Abimelech that Sarah was his sister (leaving out that she was also his wife), and the king took her into his household, but God steps in before anything irreversible happens. This shows God's protective care over his promises, since Sarah was essential to God's plan for Abraham's descendants. The dream is a mercy, really, a chance for Abimelech to correct course before he faces the consequences of what he didn't even know he was doing.