Merab was the wife of Adriel the Meholathite and the daughter of King Saul of Israel and Ahinoam.
Saul disobeyed and displeased God, and God had His prophet Samuel appoint David as the new King of Israel, although Saul and the people of Israel did not yet know of this. God facilitated David's entrance into Saul's company as a harp player. David slew the Philistine champion Goliath in one-on-one combat and became beloved by the people of Israel.
Saul grew jealous and distrustful of David. He attempted to arrange a marriage between Merab and David, only asking that David fight for him in exchange.
Then Saul said to David,Behold I will give you my older daughter Merab as a wife; only be brave for me and fight the Lord’s battles.For Saul thought,My hand shall not be against him, but let the hand of the Philistines be against him.— 1 Samuel 18:17 (AMP)
Yet David refused, humbly telling Saul that it was beyond his station. Saul then had Merab marry Adriel the Meholathite instead.
Saul tried the same scheme a second time, this time using his younger daughter Michal as bait. David agreed to the arrangement with Saul this time and set off to obtain the price demanded by Saul: a hundred foreskins from dead Philistines. Saul hoped that David would die in the attempt, but David defied Saul's expectation and returned alive and with two hundred foreskins. Thus David won the hand of Michal and became Saul's son-in-law and Merab's brother-in-law.
Merab bore five sons to Adriel, but David, after becoming king, handed her sons over to the Gibeonites in order to satisfy the Gibeonites' demands for retribution for Saul's slaughter against them. The Gibeonites then hanged them, along with Armoni and Mephibosheth, two of Saul's sons by Saul's concubine Rizpah.