Fort Seybert was constructed on a bluff just west of the Sweedlin Valley Road approximately six miles northwest of present Brandywine, Pendleton County. The first fort there was built in 1756 as a defense against Indians for the settlers inhabiting the South Fork Valley. A second Fort Seybert was built in 1758.

In April 1758, during the French and Indian War, Delaware and Shawnee warriors led by the war chief Killbuck surrounded the fort after capturing a woman and boy and killing a hunter outside the walls. They demanded that those inside surrender. The settlers deliberated a short time and agreed. Just before the gate was opened, a gun was fired at Killbuck from inside the fort, but its aim was deflected by another settler. When the settlers exited the fort, Killbuck greeted Capt. Jacob Seybert, their leader, by knocking out most of his front teeth with the butt end of a tomahawk. This action prompted the already frightened settlers to flee. Six to eight women and children and one man escaped and took refuge in forts in the Shenandoah Valley to the east. Nonetheless, the Indians succeeded in capturing 41 of the settlers.

After looting and setting the fort on fire, the warriors began separating the prisoners into two groups. Seventeen older adults were killed, including all the men. A bedfast Hannah Hinkle perished in the flames of the fort. The remaining group of 24 women and children were forced to return with the Indians to their villages in the Ohio country. Eventually, most of the captives were able to return from Indian captivity. Two or more of the smaller children remained with the Indians and at least three others died in the Indian villages. An Indian attack on nearby Fort Upper Tract occurred within days of this incident.

A fort was rebuilt at the site after the 1758 massacre, known as Blizzard's Fort after the property was sold to the Blizzard family in 1768. This third fort may have remained in use through the Revolutionary War period.