Nearly five years prior to Common Unification, Iodun Myrdinn had united much of the continent under one banner. He set his sights north to conquer Scycaen Imperium, which had consolidated under the rule of the Terrescel family in response to Common League expansion. Occupied with eastern expansion, Iodun tasked his northern general Yilmaz Mylenthalas to lead the battle against Riyissan Terrescel and take the Wyrmhorn Reach.
The resulting battle was the largest ever recorded between two largely-Human armies. In an initial skirmish pushing up to the Snoutridge, Mylenthalas' army overwhelmed a prominent Scycaen camp stationed south of the Sylphglade. Larger in number and drilled well by their leader, they soundly defeated their smaller foe, who almost immediately broke rank and retreated north into the forest.
Though they were outnumbered, the next battle was ultimately a resounding victory for Scycae. While following their foes into the woods, the Common army stayed uniformly compact, not wanting to have battle communications interrupted by the trickster Fey of the Sylphglade. The density of their formation allowed a small Scycaen garrison, led by Riyissan Terrescel himself, to slip past them undetected. Having made a deal with Fairy allies, Terrescel's garrison rounded the glade using a hidden Fey Crossing and were able create a pincer at the regiment's rear.
This maneuver created chaos; fighting on two fronts in a dense and magical forest, the Northern army was tactically overwhelmed. Before they could reach Scil-Torrenae, their attentions were split, and not enough of their force was able to commit to the assault. They were forced to retreat from the Sylphglade but lost many of their number in doing so, and the Imperium's forces followed them out of the woods into the rocky southern field.
This last battle ended when an enormous blast from Terrescel's Soulflare Sword Scyseti struck a nearby mountain. The mountain split in twain from the forces, creating what became known as Scyseti's Cut, and the remaining Common forces completely lost morale at the sight of such power. Mylenthalas ordered his army to stand down, and the last of his decimated forces were captured.
The conclusive loss at the Battle of Sylphglade permanently halted Commona's northern expansion. Scycae began to build further mountain defenses at Scil-Torrenae to further ensure that ground attacks were all but impossible, and the Common League declared Unification without the northern nation. A treatise between the two was established nearly four years afterward, agreeing the borderline was placed at the "Ironhold".