I Samuel - Chapter 22
Saul Goes Too Far
David left Gath and fled to Adulam. His family heard and joined him there. He was also joined by a variety of equally troubled and oppressed people who identified with him, 400 in all. David went to the King of Moav and asked if his parents could stay there. (His logic was that they were descendants of Ruth, who was from Moav.) Gad the prophet told David to return to Judah.
Meanwhile, Shaul was appealing to his fellow Benjaminites to join him against David. He called for loyalty and his lackey Doeg seized the opportunity. He reported that he had seen David in Nov and that the Kohanim (Priests) of the city had given supplies and Goliath's sword to David. This infuriated Shaul, who accused Achimelech of siding with his enemies against him. Achimelech replied that, as far as he knew, David was Shaul's loyal son-in-law and that he would never act against Shaul. Shaul paid no heed. He ordered the city destroyed and the Kohanim killed. His men were hesitant to carry out this order, so Doeg took it upon himself. 85 men, plus women, children and livestock were killed. (You will notice that Shaul acted here as he was supposed to act towards Amalek earlier. The Talmud tells us in Yoma 32b that one who is merciful when he should be harsh will ultimately be harsh when he should be merciful.)
The only survivor of the massacre in Nov was Achimelech's son, Evyasar. He ran to David, who took full responsibility for the atrocity, since he had seen Doeg and should have realized that something like this would happen. David commemorates this incident in Psalm 52.