The duel between Arthur and Odin occurred in the woods. When Princess Mithian asks Arthur for help in getting her father, King Rodor, back, he didn't think it would be a trap set by King Odin and Morgana. Arthur had killed Odin's son a long time ago and Odin never forgave him. In a fit of revenge, Odin killed Arthur's father, Uther (The Wicked Day). He even sent an assassin to kill Arthur (The Once and Future Queen).
Arthur, Merlin, Mithian, Gaius, Morgana disguised as Hilda, and some of the Knights of the Round Table go on a quest to rescue Mithian's father who is thought to be hiding in a cave in the woods. Instead, Rodor is being held by Odin who is hoping Arthur will take the bait. Once they arrive at the cave where Rodor is captive Arthur is confused for a moment asking Mithian where her father is. It doesn't take him long to realise that it was a trap.
Percival and Arthur, along with the rest of his knights outside the cave, briefly get captured by Odin and Morgana. But with the help of Merlin and Gwaine, they all manage to flee. Merlin and Arthur split up from the rest of the group to try to create a distraction.
Arthur and Merlin are eventually cornered by Odin and his men. Odin tells his men that he wants to kill Arthur himself and Arthur and Odin start to fight to the death. Arthur eventually has Odin with his sword to Odin's throat. Merlin stops Arthur from killing Odin, telling him that killing Odin won't achieve anything. Arthur tells Odin that this is no answer and that this would cause a war with no end. Odin counters this saying he believes there is no other way. Arthur offers a truce between their two kingdoms. Although at first unwilling, Odin eventually agrees to Arthur's terms (Another's Sorrow).