Dagmer Cleftjaw is the ironborn captain of the longship Foamdrinker. He trained young Theon in arms. Dagmer is known to be one of the deadliest warriors of the Iron Isles. After raiding the Stony Shore with Theon, Dagmer was convinced to overtake Winterfell. Dagmer attacked Torrhen's Square as a diversion to lure Rodrik Cassel away from Winterfell. Dagmer lost Torrhen's Square to Cassel when he returned, but it's said he went back & re-took it again & continues to hold it.