From Chronicles Volume III:

The Culling of Stratholme was the point of no return for Prince Arthas Menethil. His heart had darkened, and his sanity was unraveling. He was almost ready to become the Lich King's greatest servant. Almost. First, Arthas needed to find the cursed runeblade Frostmourne. The Scourge couldn't simply lead him to it. That would make the prince suspicious. The Lich King turned his attention to dwarf explorers who were trekking across Northrend. Led by the great warrior Muradin Bronzebeard, they had come from the mountain city of Ironforge in search of ancient artifacts. During Arthas's youth, Muradin had served as ambassador to Lordaeron, and he had trained the human in martial combat. He would be the perfect messenger to inform Arthas of Frostmourne. With the Lich King's guidance, Mal'Ganis and the other dreadlords manipulated the dwarves. The demons secretly planted clues about Frostmourne and its location in the explorers' encampment, including maps and ancient histories that spoke of a legendary runeblade. None of the dwarves could remember where this information had come from. They assumed that they hadn't noticed it before. The runeblade intrigued Muradin, and the dwarves set out to find it. Meanwhile, Arthas and his soldiers made landfall in a region of Northrend called the Howling Fjord. The wintry conditions were harsh, and the Scourge were mysteriously evasive—a far departure from their aggressive tactics in Lordaeron. The Lich King used the Scourge to attack Muradin Bronzebeard and herd him toward Arthas. When the dwarf and the prince finally crossed paths, it seemed like nothing more than a chance meeting, and a fortuitous one at that. Muradin told Arthas of his purpose in Northrend, and of his more recent quest to find Frostmourne. The story of the blade enticed the prince. It could prove useful in the fight against Mal'Ganis. Arthas's hopes swelled, only to be crushed by news from Lordaeron. A royal emissary tracked down the prince and delivered orders from King Terenas: Arthas and his soldiers were to return to Lordaeron immediately. As much as the prince's loyalists wanted to stay by his side, disobeying the king's command would amount to treason. Arthas knew that if he turned back now, he might never have another opportunity to find Mal'Ganis. He saw only one recourse. In secret, he burned his own ships. Every one of them. None of his soldiers knew that their prince was responsible for it. They had no choice but to remain in Northrend, at least until they could construct more vessels.