Abiathar son of Ahimelech the son of Ahitub, Abiathar the priest
And to Abiathar the priest the king said, “Go to Anathoth, to your estate, for you deserve death. But I will not at this time put you to death, because you carried the ark of the Lord God before David my father, and because you shared in all my father's affliction.” - 1 Kings 2:26
A priest in King David's court (2 Samuel 20:25) and King Solomon's court (1 Kings 4:4). He fled Saul with David and offered his ephod for David to seek God's guidance. He supported Adonijah, not Solomon, when he was trying to take the throne after David. (1 Kings 1:7) He was deposed by Solomon in favor of Zadok (1 Kings 1:26-27,35) in accordance with Yahweh's denunciation of Eli's family. (1 Samuel 2:27-36)
(The ESV Study BibleTM, English Standard Version (ESV) by Crossway Bibles, 2007. Page 1369)
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His Life
Abiathar fled to David as Saul killed all priests
When Saul ordered the deaths of all the priests for "aiding David's rebellion," Doeg killed almost all the priests and also the people and livestock at Nob, but Abiathar escaped and fled after David. He told David about the slaughter, and David told him he knew it would happen when he saw Doeg. He told Abiathar to stay with him and not be afraid, for he will protect him. (1 Samuel 22)
David uses Abiathar's ephod to know to flee Keilah
Abiathar fled to David to Keilah with an ephod in his hand. David knew Saul was plotting harm against him, so he asked Abiathar for the ephod to be brought to him. He asked if the men of Keilah would surrender him into his hand and whether Saul would come down as his servant had surely heard. The LORD affirmed Saul would come down and that Keilah would surrender him. They fled Keilah for Ziph, Maon, and Engedi. (1 Samuel 23)
David uses Abiathar's ephod to know to overtake the Ziklag raiders
David and his men found Ziklag burned with all its people taken captive. David told Abiathar to bring the ephod, and David inquired, "Shall I pursue after this band? Shall I overtake them?" God affirmed that he would overtake and rescue. Indeed, his men rescued a dying Egyptian who led them to the band, and David killed all of them apart from 400 camel riders and recovered everyone and every single thing and all the flocks and herds the Amalekites had. (1 Samuel 30)
Abiathar, Zadok, and the Levites bring the ark to David as he flees Absalom, but David has them stay in the city to be his eyes and ears
When King David was fleeing Absalom, Abiathar and Zadok came to him with all the Levites, bearing the ark of the covenant. They set down the ark until the people had all passed out of the city. King David told the priests to return to the city with the ark and their sons. He said he will wait at the fords of the wilderness until they send word. (2 Samuel 15:24-29)
Hushai sends word to David to flee Absalom
Hushai came to Zadok and Abiathar with a message for David: Ahithophel advised Absalom to let him take out only David with 12000 chosen men and to capture the rest, while Hushai advised for Absalom to personally lead all Israel to take out David and his men. Absalom and all the elders of Israel found Hushai's counsel better. Therefore, he wanted David to know, "Do not stay tonight at the fords of the wilderness, but by all means pass over, lest the king and all the people who are with him be swallowed up."
Jonathan and Ahimaaz waited at En-rogel, for they were not to be seen entering the city. A female servant was sent to tell them, and they got the message to David.
David sways the hearts of Judah through Zadok and Abiathar, and he replaces Joab with Amasa as commander
David told Zadok and Abiathar to tell the elders of Judah, "Why should you be the last to bring the king back to his house, when the word of all Israel has come to the king? You are my brothers; you are my bone and my flesh. Why then should you be the last to bring back the king?" and to Amasa, "Are you not my bone and my flesh? God do so to me and more also, if you are not commander of my army from now on in place of Joab." Judah sent word to the king, “Return, both you and all your servants.”
Abiathar supports Adonijah as king
Abiathar supported Adonijah when he was exalting himself, saying "I will be king," during King David's old age. He attended the sacrifice at Serpent's Stone near En-rogel and ate, drank, and proclaimed, "Long life King Adonijah!" After the feast, there was a noise. Jonathan the son of Abiathar the priest came and told Adonijah's supporters about Solomon's coronation, and all the guests trembled and rose and went their own ways.
Abiathar's exile
After putting Adonijah to death for asking for Abishag as his wife, Solomon said to Abiathar, "Go to Anathoth, to your estate, for you deserve death. But I will not at this time put you to death, because you carried the ark of the Lord GOD before David my father, and because you shared in all my father's affliction." So Solomon expelled Abiathar from being priest to the LORD, thus fulfilling the word of the LORD that he had spoken concerning the house of Eli in Shiloh. (1 Kings 2:26-27)
King Solomon put Zadok the priest in his place. (1 Kings 2:35)