Hagen tells him about BrÃ 1\/4 nnhilde on the rock . In his drugged state , Siegfried offers to win a wife for Gunther , who tells him about BrÃ 1\/4 nnhilde and the magic fire which only a fearless person can cross . They swear blood-brotherhood ( Hagen holds the drinking horn in which they mix their blood , but he does not join in the oath ) and leave for BrÃ 1\/4 nnhilde 's rock .