The monastery 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 fanatical Scarlet Crusade. The crusaders are intolerant of all non-human races, regardless of alliance or affiliation. They believe that any and all outsiders are potential carriers of the undead plague - and must be destroyed.
Reports indicate that adventurers who enter the monastery are forced to contend with Scarlet Commander Mograine - who commands a large garrison of fanatically devoted warriors. However, the monastery's true master is High Inquisitor Whitemane - a fearsome priestess whose guardians seem difficult, if not impossible, to fell in battle.
The Cathedral also starts with a cloister, the Chapel Gardens; and is followed by the Crusader's Chapel, which houses the three bosses. As in the rest of SM, fleeing humanoids can easily pull lots of other mob groups. Before engaging the final bosses, High Inquisitor Whitemane and Scarlet Commander Mograine, all of the Cathedral's interior must be cleared of any other enemies, or they will come to their aid. Hunters' traps can freeze or slow Mograine and Whitemane, so you can have more time for ranged attacks. Another approach is to send either 2 tanks or combat pets to take the bosses and have everyone else be ranged and healers. Area attacks and other multi-target spells are useful. After Mograine is killed, Whitemane will come through previously locked doors and resurrect him, and they will heal each other to 100% health. For some classes, the Cathedral is soloable, starting at around level 50. It is a particularly valuable training ground for Hunters, since they can learn much about double-mob pulling and trapping here. Warlocks can use the Succubus to control humans and prevent them from running.
Scarlet Wizard - Their spells and armor are relatively weak. If it is necessary to engage both a Wizard and Centurion/Myrmidon/Champion (melee) at once, tank the melee mob and kill the Wizard first. Try not to gather around these guys since they use [Fire Shield] and will spam [Arcane Explosion] if several party members are close to them.
Scarlet Champion - A fairly minimal threat one on one, if you have good armor. However in numbers these can be dangerous, especially if accompanied by an Abbot who can heal them. Try taking them out quickly, preferably after the healers and casters since their Holy Strike is quite a pain.
Scarlet Abbot - Priority target that has [Inner Fire]. Focus fire and kill these as soon as you see them, before any other mobs. Interrupt their spells if you can, and use mana-drain spells to prevent them from healing with [Renew] and [Heal]. They don't run, but Enrage at low HP instead.
In addition, Library and Armory mobs populate the Cathedral.