The Brotherhood of the Horse (aka Knights of Azeroth or Knights of Stormwind) is the official name of the Kingdom of Stormwind's knights. They are advisors to the king and commanders of Stormwind's military.
After Logan Wrynn I was assassinated in Brightwood in 48 K.C., his son, Daniel Wrynn, founded the Brotherhood of the Horse in order to provide an order of knights loyal to the House of Wrynn over the noble houses of the realm. Many consider the brotherhood to be the progenitor of the Stormwind Army, as it stood as the kingdom's first armsmen directly loyal to the crown. The brotherhood kept peace in the realm throughout the Age of Consolidation, acting as chivalrous mediators for the newly united noble houses.
When Thomas I was found dead in his chambers in 125 K.C., the brotherhood was torn over vying claims of whether he was killed by his wife, Mary Brightwood, or his brother, Samson Wrynn. Many of the Brotherhood were exiled from Stormwind City with the latter as those who supported Mary's claim to the throne were backed by the peasantry.
As Mary tightened her grip on the kingdom, slaughtering those who protested her dissolution of the Church of Northshire, those of the brotherhood loyal to Samson Wrynn aided him in gathering support for the eventual toppling of her reign. There they followed him all the way to the Siege of Brightwood, where they fought valiantly and quashed the rebellion of House Brightwood once and for all.
To honor their dedication and service to the crown, the royal family under Samson Wrynn funded the construction of Karazhan Keep - the tower of the Brotherhood. The order's bastion of power and its surrounding buildings show their influence, such as horse heads in various ornamentation. From then on, the Brotherhood of the Horse steadfastly defended the realm, aiding the House of Wrynn in expanding its borders and defending it from those who would threaten the realm.
At the onset of the orcish wars, the Brotherhood of the Horse fought valiantly in defense of the kingdom. Outnumbered and overwhelmed, the knights manning Karazhan were slaughtered. The fallen sorcerer Medhiv took residence in the seat of Stormwind's knighted power, directing his orcish minions to ravage the landscape. Most of the knights were killed fighting in the last battles of the war, but took many orcs with them. Many of the knights who fell to the orcs were transformed into death knights by Gul'dan.
One of the most recent notable members of the brotherhood was Anduin Lothar, who became a knight champion. After the The First War, in recognition of his services to his people, Sir Lothar was given the title of Regent Lord of Azeroth. Rallying the people of Azeroth across the sea to Lordaeron, Lord Lothar brought about the Alliance of Lordaeron that contended with the Orcish Horde during the The Second War. The remaining knights of the Brotherhood of the Horse fought alongside the armies of the Alliance of Lordaeron, offering their expertise in orcish combat.
After the Horde's mysterious retreat from Lordaeron and Grand Admiral Proudmoore's victories at sea, Lothar led the armies of the Alliance to liberate much of Azeroth and Khaz Modan (meeting up with Muradin and Brann Bronzebeard), eventually breaching the Black Morass itself. Lothar was killed at the foot of Blackrock Spire after a force of Alliance troops he personally commanded was ambushed by the Horde forces.
After the orcish wars, the brotherhood has slowly recovered. Still in service to the crown, the knighted order of Stormwind adheres to the tenants set by the knights who gave their lives in defense of the kingdom.
As the kingdom of Stormwind progresses into a new modern age, the membership of the brotherhood is no longer restricted to plate-wearing mounted warriors. A number of mages, rangers, and even citizens of great utility to the kingdom have been knighted as honorary members of the brotherhood. The Brotherhood of the Horse adheres to a time-honed tradition of apprenticeship and respect for elders. As such, the ranking of the order is bound by experience and merit.
Once inducted into the Brotherhood of the Horse, knights must show gallantry and loyalty beyond their peers to ascend in rank. Only the most experienced of knights ascend into these sought-out positions, and respect to their wishes is expected of the brotherhood.
Knighting by Apprenticeship
A page is an attendant to a knight of the brotherhood; an apprentice squire. Pages serve for seven years, serving, cleaning, and learning the basics of combat, and the knight he was working for is expected to treat him fairly. The knight is expected to give the page private combat training from the age of seven (after starting cutting hair) until he reaches the age of fourteen.
At age fourteen, the page could graduate to become a squire. As part of his development to that end, he serves an existing knight as an attendant or shield carrier. The squire sometimes carries the knight's flag to battle with his master.
If the squire proves his loyalty in battle, he would have a dubbing, an official ceremony to become a knight of the brotherhood.
Knighting by Merit
On rare occasions, warriors or citizens of the kingdom are recognized for their devotion to the king through knighting by merit. Through glorious battle or utility to the crown beyond measure, individuals knighted by merit are taken into the brotherhood's open arms.