Step 1: Get to the Icecrown Citadel instance, which is located at the stairs West of the Wrathgate in Icecrown. Step 2: You need to defeat all the bosses of each of the wings (Saurfang, Sindragosa, Professor Putricide, Blood Queen Lana'thel) to activate the teleporter. Step 3: The teleporter in the middle of the instance (between the entrances of each wing, suspended from chains and with Val'kyr flying around it) leads to the Frozen Throne. Note that in order to be able to attack the Lich King in hard mode, you need to also have defeated each of the wing's End bosses on hard mode.
ICC is an abbreviation for Icecrown Citadel. Icecrown Citadel is a raid in World of Warcraft. It is categorized in a 10 or 25 man raid. As you enter there is an army waiting for you. There is your chance to stop and cast summons. First, you kill the trash mobs, then after that is Lord Marrowgar. This is typically a boss fight raid. Next is Lady Deathwhisper, Gunship Battle, Deathbringer Saurfang, Festergut, Rotface, and Professor Putricide (all different times). Then there is the Blood Prince Council, made up of Keleseth from Utgarde Keep, Prince Valanar, and Taldaram. Next we've got Blood-Queen Lana'thel. Then here comes the dragon fights in Frostwing Halls with Valithria Dreamwalker and Sindragosa. The Lich King is next as being the last boss. Beating him will earn the title, The Kingslayer.
ICC is an abbreviation for Icecrown Citadel. Icecrown Citadel is a raid in World of Warcraft. It is categorized in a 10 or 25 man raid. As you enter there is an army waiting for you. There is your chance to stop and cast summons. First, you kill the trash mobs, then after that is Lord Marrowgar. This is typically a boss fight raid. Next is Lady Deathwhisper, Gunship Battle, Deathbringer Saurfang, Festergut, Rotface, and Professor Putricide (all different times). Then there is the Blood Prince Council, made up of Keleseth from Utgarde Keep, Prince Valanar, and Taldaram. Next we've got Blood-Queen Lana'thel. Then here comes the dragon fights in Frostwing Halls with Valithria Dreamwalker and Sindragosa. The Lich King is next as being the last boss. Beating him will earn the title, The Kingslayer.
Frostmourne is not available as a weapon in World of Warcraft without being on a private server, where a fascimile might be spawner. Shadowmourne, the legendary player equivalent for Death Knights, Warriors and Paladins, is obtainable through a long questline starting in Icecrown Citadel on 25man. This quest first requires obtaining the shaft of Arthas' old mace (to be picked up in the icy cave where Frostmourne was first discovered) and a number of Orbs (obtained with Justice points or dropping from ICC bosses). The starter quest is available directly after zoning into ICC, from an NPC nearby. After that, you must perform various other tasks including killing a number of bosses, performing special feats during boss fights (like getting breathed on by Sindragosa) to charge the weapon. It will become increasingly more powerful until the final step - defeating the Lich King. Once this is done you will have the legendary weapon Frostmourne. Finding this weapon on WoWHead or WoWpaedia will also link through to the entire questline required to get it.