So Abraham prayed unto God: and God healed Abimelech, and his wife, and his maidservants; and they bare children. Genesis 20:17

Even though Abraham was considered righteous before God because of his faith, he was not a perfect man. Here we see as Abraham was travelling through Negev, Abraham lied to King Abimelech that Sarah was his sister and not his wife. Why? Because Sarah was beautiful (Genesis 12) and Abraham feared he would be killed so the King could take his wife. This was not the first time Abraham has done this

So King Abimelech did take Sarah for a wife , as he thought she was just Abrahams sister. However because he did this, it put a curse on the Kings entire family . (Genesis 20:6)

God revealed to the King in a dream that Abraham was a prophet and he should return Sarah immediately to him or his entire family will die. Why was this so important to God? Remember Sarah at this time was pregnant with Isaac , the promised Son (Genesis 18). God probably wanted to preserve the sacred line of Abraham with no corruption

God healed or restored the curse that was put on King Abimelechs family. Notice how God used Abraham as a mediator to request the Kings curse be lifted. This shows a few things

1. Even though Abraham was held in high regard to God, he was not a perfect man. But by his faith in God he was considered righteous (Genesis 15:6)
2. The family line of Abraham was obviously held in high regard by God, to the point anyone who interfered would be cursed. This was because the Messiah (Christ) would eventually come from the line of Israel
3. Notice God made a revelation in a dream to a wicked man like King Abimelech. Gods sovereignty and will is accomplished no matter who God needs to be used

Thy kingdom come. Thy will be done in earth, as it is in heaven. Matthew 6:10