Bremen was the ancient leader of the Druids. He bid to create the Sword of Shannara to confront Brona centuries ago. His spirit is bound to earth and exists in Hadeshorn.
Allanon communicated with his shade to get advice regarding the return of Brona.
He was summoned again so with his ancient nowledge, combined with the magic of items of power from the Four Lands, he restored the Sword.