The Crimson Storm is a revived alliance between the Forsaken and the Sin'dorei forces that re-took Silvermoon from the Scourge and has been re-taking Quel'thalas from Dar'khan's control steadely ever since. Their enemies lie partially in Alliance aligned humans from Stormwind aswell as the Kaldorei, both of which would gladly see the downfall of the Forsaken and the Sin'dorei.
The main enemies of the Crimson Storm are the Scourge - With some Sin'dorei harnessing special hatered towards the San'layn, and some (most) of the Forsaken harbouring great hatered towards Arthas Menethil.
The Crimson Storm has their main headquarters - not in Silvermoon, but in Tranquilien and in Lordaeron. When Tranquilien was attacked by the Kaldorei during an earlier Road To Supremacy RP-PvP campaign during the Dorei Wars (War of the Children), the Tauren Warlord Armos Taisha'ishte answered the Forsaken and the Sin'dorei's plea for help and so brought a legion of Horde volunteers to Quel'thalas to defend the base.
Since that day, the Forsaken and the Sin'dorei forces stationed at Tranquilien has followed the Horde into a great deal of battles, including the invasion of Aerie Peak and the Battle for Warsong.
At the Defense of Warsong Gulch, however, the Horde suffered tremendous defeat - Partially due to the interference of Dalarani maguses. This defeat shattered the alliance between the Forsaken and the Sin'dorei and made both races withdraw from the Horde offensive to tend to their own lands.
NOTE: This does not mean the Sin'dorei and Forsaken race as a whole. It would be ludicrous for anyone to claim that they speak for an entire race - The above statement refers only to the Forsaken and Sin'dorei roleplayers who roleplayed as part of this offensive force.
Months later, the secretive and dangerous order of the Iron Ring set into motion a political underground campaign to ressurect the alliance between the Forsaken and the Sin'dorei. The Preceptor Lord, Archbishop Abraham Tremayne and the Kul'tiras noble, Lord Shavrash Aramain were the primary puppeteers pulling the strings within the Royal Council of Silvermoon (also known as the Convocation) and saught out powerful and influential Sin'dorei and Forsaken with enough power and personality to spark the alliance back into life. The Iron Ring found their puppets in Warlord Antus Draconus and Magistrix Poesi Verdande and made sure that fate would them meet.
The Iron Ring played on the Forsaken and the Sin'dorei's thirst for vengeance to fuel their enthusiasm, and so the Forsaken-Sin'dorei alliance was reborn - under the name of the Crimson Storm. The official banner for the Crimson Storm was a red L - The mark of Lordaeron, adorned with bloodied swords and graced by Phoenix Wings on a black tabard.
The true motive of the Iron Ring's manipulation of events can be discussed greatly, for the order is secretive. Those few who are aware of the fact that they are being manipulated, however, assume that it has to do with things greater then the conflict of a single Kalimdor outpost - Indeed, most eyes turn to Dalaran.
The interferenace of neutral Dalarani ambassadors at the Defense of Warsong was seen as an insult to many Forsaken and Sin'dorei of which many were former citizens of Dalaran. A campaign was launched towards Nethergarde Keep in the Blasted Lands to punish the Alliance aligned Dalarani and force the Violet Citadel to acknowledge the Sin'dorei and the Forsaken as equal citizens within the city.
The Crimson Storm demanded that atleast one Forsaken or Sin'dorei magus be accepted into the Six as a ruling member of the city.
Although the battle of Nethergarde led to crushing defeat to the Crimson Storm - their goals had been accomplished. The Arch Mage, Aethas Sunreaver - Who had no affiliation with the Crimson Storm what so ever, rose to become a member of the six - and many Sin'dorei and Forsaken citizens began their exodus to Northrend - to return home to the Violet City.
However, the Crimson Storm still stirs...
Credits go to Poesi.