according to wiki brackens are also first man like blackwoods, just a stupid feud over who was the king in the area and who was the vasal.
While I have been unable to find the source of the disappearance, I have found out that Lord Tytos Blackwood feared for his family and even Lord Jonos Bracken marshaled his army to help Raventree Hall but after much deliberation between the two lords, both surrendered their castles to House Tully and left for North.