at no other place in the last tales is the subsequent transformation more remarkable than in this , the precursor of the story of the capture of beren and felagund and their companions by sauron the necromancer , the imprisonment and death of all save beren in the dungeons of tol-in-gaurhoth ( the isle of werewolves in the river sirion ) , and the rescue of beren and overthrow of sauron by luthien and huan .