Northeastern Tirisfal Glades
The Tirisfal Monastery, now known as the Scarlet Monastery, was one of the last bastions of the zealous Scarlet Crusade, with their other held territories (Hearthglen, Tyr's Hand, the Scarlet Enclave, the western portion of Stratholme, etc.) having fallen. It was once a proud bastion of Lordaeron's priesthood - a center for learning and enlightenment. With the rise of the Undead Scourge during the Third War, the peaceful Monastery was converted into a stronghold of the Crusade. It is located in the northeastern corner of the Tirisfal Glades, and the garrison was commanded by Renault Mograine, son of the legendary Ashbringer Alexandros Mograine, and Sally Whitemane. While Mograine would be slain, Whitemane continued to command the monastery alongside Commander
Durand, until they too were slain by Lillian Voss. Following this, the Monastery was led for a final time by High Commander Goodchilde, until he and the inhabiting crusaders were wholesale butchered by the Knights of the Ebon Blade.
It served as the staging ground for the Alliance during the Siege of Tirisfal, but was abandoned following the battle.
The original Monastery was presumably built sometime prior to the First War. As a center of Lordaeron's priesthood, it was a frequent destination of religious scholars seeking to contemplate the Holy Light. The sizable Monastery also served as a defensive structure, bearing machicolations upon its battlements, and contains a fully stocked armory.
Scourge of Lordaeron
As Lordaeron collapsed around the Monastery, the inhabitants did not stay castled up within the Monastery's defenses. Either by attrition, departing to help others suffering from the Scourge, or simply forced to leave to find food, the original inhabitants would trickle out from the sacred place. It was left abandoned and curiously untouched by both the Scourge and Forsaken during the Third War, perhaps due to the holy ground discouraging the undead from tresspassing.
Under the Crusade
Eventually, the Monastery was scouted out by the future Scarlet Highlord Alexandros Mograine and his companions Brigitte Abbendis and Inquisitor Fairbanks. Seeking a forward base to prepare for an eventual assault on Capital City and the Forsaken, the priests and paladins who would become the Scarlet Crusade would return to the Monastery and intensely fortify the already well-defended structure.
The newly rechristened Scarlet Monastery would become one of the Scarlet Crusade's single most important installations. Though lacking the vast population of New Avalon or the defensive fortifications of the Scarlet Bastion, the Monastery would equal them in importance as it come to assume the responsibility of training new initiates of the Scarlet Crusade, converting the remnants of Lordaeron's human population into willing zealots of the Crusade.
Three times would the Undead Scourge attempt to take the Monastery during what would come to be known as the infamous Summertime Assaults. Though the Scourge would see early victories in the first two assaults, the Crusaders would grasp the Scourge's tactics upon the third and final assault, dealing a major blow to the Scourge and ensuring the Monastery would remain in their hands.
Thrice, the Scarlet Monastery has been invaded and sacked by the forces of both the Alliance and the Horde and its leaders slain. For differing reasons and using a wide array of tactics, both Factions' eyes were drawn towards the Monastery and its High Inquisitor, Sally Whitemane.
As early as after the end of the Third War, an ex-member now affiliated with the Alliance, Raleigh the Devout, had planned the assassinations of key leaders of the Crusade in the Monastery, while the leader of the Deathstalkers, Varimathras, had ordered the deaths of the leaders in the Monastery. Both factions sent Azerothian Heroes to the Monastery to eliminate the Crusade's leadership.
Some years later, the Alliance had dispatched numerous Azerothian Heroes to assist a splinter-faction rebelling against the Crusade at the same time the Horde had enlisted its own heroes to assist Sylvanas Windrunner's forces infiltrate and destroy the Crusaders; the leader of this rebellion was a Crusader named Joseph the Awakened---who would later become known as Joseph the Crazed and later, Joseph the Insane---who had claimed to have witnessed corruption after being denied the title of "Champion" and wished to reorganize the Crusade. Meanwhile, Dark Ranger Velonara had coordinated the Forsaken forces with similar objectives.
It is unclear which faction defeated James Vishas, Bloodmage Thalnos, Houndmaster Loksey, Arcanist Doan, Champion Herod or High Inquisitor Sally Whitemane or had looted the various artifacts and documents attributed to the Crusade such as the Compendulum of the Fallen, a roster of living and deceased Crusaders, but Commander Mograine was slain by the ghost of his father Alexandros when his younger brother Darion travelled to the Monastery with the corrupted Ashbringer. Additionally, Vishas eluded death and later became a gladiator for the Brawler's Guild.
However, the "victory" over the Scarlet Monastery would prove hollow; according to various Heroes, a Hooded Crusader (who was actually Lilian Voss, a well-known Forsaken deserter and raised Scarlet) had spoken of the Scarlets returning to life by means of a "mass resurrection" spell by Whitemane herself. While some officers, lieutenants and even Commander Mograine were unable to be resurrected, the Scarlets had returned to the Monastery in greater force and had even picked up Pandaren Monasticism from a Human named Korloff.
Similar to the previous invasion and sacking, such heroes were tasked with both the seizing of documents and relics and the defeat of the new leaders: Flameweaver Koegler, a newly-risen Thalnos, Houndmaster Braun, Armsmaster Harlan, Brother Korloff and Commander Durand. Using two enchanted blades found within the Monastery, heroes had plunged them within Whitemane's corpse---preventing her from being resurrected---before Voss had revealed herself and took the blades.
Unbeknownst to Voss or the heroes accompanying her, the League of Lordaeron, the Clergy of the Holy Light, the Scarlet Hammer and the Grey Blades had launched an offensive against the Monastery---killing other leaders and discovering the higher leaders (including Whitemane) dead within the burning ruins.
Revival and Final End
The Scarlet Monastery became the stomping grounds for many smaller Scarlet groups, who would rise and fall within the walls of the once sacred place.
Primarily among them were crusaders led by High Commander Goodchilde, who was in the process of revitalizing the Crusade at the time of the Burning Legion's invasion of Azeroth. In need of a third horseman, the Ebon Blade invaded the Monastery under the orders of the Deathlord and the Lich King. By the orders of Darion Mograine, the Ebon Blade slaughtered every single scarlet within the Monastery, few escaping to the hills.
By the end of it, the only scarlets remaining were those who had fled in time; otherwise, every scarlet was dead within, leaving the Monastery undermanned and hollow.
It would later serve as the staging ground for the Alliance during the Siege of Tirisfal, as the holy site had not yet been defiled or taken over by the Forsaken. What Scarlets remained either were captured, converted to the Alliance, or once again had fled. With the monastery as their mountain fortress, the Alliance successfully sieged Tirisfal and destroyed Brill, which became the new forward command with the arrival of Kings Genn Greymane and Anduin Wrynn.
Following the destruction of the Undercity and the blighting of Tirisfal, Alliance forces, what few remained, withdrew from the area for a safer operation in the now Bloodfang-Controlled Fenris Keep.
Adapted from Wowpedia.