In the show: After taking over Tumbleton with the Hightower army, Ormund conspires with Ulf the White (Tom Bennett), one of Rhaenyra’s sworn dragonriders, to betray the queen. Ormund promises that once the war is over, Ulf can take over Driftmark and become Lord of the Tides.
Ormund is probably lying, and anyway, Ulf is too unruly and self-interested to be trusted with a battle plan.
When Rhaenyra’s army arrives to retake Tumbleton, Ulf gets drunk and passes out in a privy. Ormund finds him mid-battle and berates him, calling Ulf a “useless, baseborn stump.”
Once Ulf finally takes to the sky on his dragon, Silverwing, he decides to betray both Rhaenyra and Ormund.
Ulf burns Rhaenyra’s army in the field while Ormund is attacked by Ser Roderick Dustin (Tommy Flanagan), the Lord of Barrowton, known to history as Roddy the Ruin.
Roderick commands a group of Northern soldiers known as the Winter Wolves, and he’s determined to die fighting for the dragon queen. Despite losing an arm, he overpowers Ormund in one-on-one combat and stabs him from behind.
Ormund, bleeding out, lives just long enough to watch Ulf turn Silverwing toward the walls of Tumbleton. Both he and Roderick are burned alive by dragonflame.
In the book: Ormund is killed by Roddy the Ruin during the Battle of Tumbleton. There’s no mention of Ormund striking a deal with Ulf, or having anything to do with Ulf’s violent defection:
“In the chaos that ensued, the northmen fought their way through ten times their own number to where Lord Ormund Hightower sat his warhorse beneath King Aegon’s golden dragon and the banners of Oldtown and the Hightower. As the singers tell it, Lord Roderick was bloody from head to heel as he came on, with splintered shield and cracked helm, yet so drunk with battle that he did not even seem to feel his wounds. Ser Bryndon Hightower, Lord Ormund’s cousin, put himself between the northman and his liege, taking off the Ruin’s shield arm at the shoulder with one terrible blow of his longaxe…yet the savage Lord of Barrowton fought on, slaying both Ser Bryndon and Lord Ormund before he died. Lord Hightower’s banners toppled, and the townsfolk gave a great cheer, thinking the tide of the battle turned.”