Icemourne was an unobtainable weapon, added in the 1.2 version. It was entirely removed in the 1.2.4 version.
Even when it could be obtained with hacks or inventory editors, Icemourne was never a usable weapon. Although its name, sprite, and overhead swinging animation did work, it had no statistics, hence trying to use it as a weapon would not affect the enemy. The item tooltip referred to it as a placeable object, but when trying to do so it would turn into an Angel Statue.
This sword's name and appearance is a reference to Frostmourne, a sword wielded by the former Lich King, Prince Arthas Menethil, from the Warcraft franchise. It was likely removed because it would violate the copyright agreement of Blizzard Entertainment.