This is a guide to get the new legendary Shadowmourne from the new 25 man raid instance Icecrown Citadel.
Link to Shadowmourne Model/Stats: http://img94.imageshack.us/img94/3950/shadowmourne.jpg
The quest to attain Shadowmourne starts with Darion Mograine. The quest is The Sacred and the Corrupt - Quest - World of Warcraft . To start this quest you must first be friendly with the new Icecrown Citadel faction, the Ashen Verdict. You get Ashen Virdict rep from quests, trash kills, and boss kills in ICC and the three 5 man dungeons. All quests from here on must be done in 25 man ICC.
To complete this quest you must:
Gain twenty-five Primordial Saronite - Item - World of Warcraft Which is the new Crusader Orb of 3.3, it can drop off bosses in ICC, or bought with Emblems of Frost, the Emblem of Triumph of ICC. Your best bet is to purchase these from people and buy them yourself with alts by doing ICC 25/10 and the Randomly daily dungeon and the new weekly raid quest to gain Emblems of Frost.
Next you get the quest item Light's Vengeance - Item - World of Warcraft Which is found in Frostmourne Cavern in Dragonblight. [ame=http://www.youtube.com/watch?v=uPQR_BkDDM0&annotation_id=annotation_915881&feature=iv]YouTube - 3.3 PTR The Sacred and the Corrupt QUEST[/ame] This video shows you how to do the quest.
Next up you must gain quest items off two bosses in ICC Festergut's Acidic Blood - Item - World of Warcraft and Rotface's Acidic Blood - Item - World of Warcraft . Blizzard has stated that the wing of ICC which these bosses are found in will open a couple weeks after 3.3 is released (link to that post here: World of Warcraft - English (NA) Forums -> Icecrown Citadel Raid Access Progression )
After you attain all these items, you go back to Darion Mograine and turn them in, and you will be rewarded with Shadow's Edge http://img175.imageshack.us/img175/7839/shadowsedge.jpg
Once you have Shadow's Edge the real fun begins!
First do Unholy Infusion - Quest - World of Warcraft which you have to fight the final boss of the Plagueworks, Professor Putricide and absorb Mutated Slime and be transformed into an Abom THEN slay the Professor to complete the quest.
Next up is Blood Infusion - Quest - World of Warcraft , which you have to fight the final boss of The Crimson Hall, Blood-Queen Lana'thel. You must have Blood Mirror - Spell - World of Warcraft cast on you in phase one of the fight then defeat her to complete the quest.
Next is Frost Infusion - Quest - World of Warcraft , whih you have to fight the final boss of The Frostwing Hall, Sindragosa. To complete the quest you must get hit by Frost Breath - Spell - World of Warcraft four times while wielding Shadow's Edge then defeat her.
After you complete those three quests you must next do The Splintered Throne - Quest - World of Warcraft and collect sixty Shadowfrost Shard - Item - World of Warcraft which drop off bosses in 25 man ICC. Its not yet known if it's a 100% drop or if one boss can drop multiple shards or if it drops for only one person or multiple people in the raid thats on the quest. This will most likely be the bottleneck so shadowmourne cannot be farmed by multiple people in the same raid.
After all of this you must do A Feast of Souls - Quest - World of Warcraft Which is simply kill 1,000 mobs while wielding shadow's edge in ICC, this is the only quest which can also be done in 10 man ICC.
CONGRATULATIONS YOU ARE NOW THE PROUD OWNER OF SHADOWMOURNE!!