Battle of Bruma

The Battle of Bruma was a massive battle that took place in the beginning of the Fourth Era. It was fought between the daedric hordes of Mehrunes Dagon and the armies of the Empire (joined by Ra'rgo and his Knights of the Nine) in order to bait the Mythic Dawn into opening a Great Gate so that the Blades could find a Great Sigil Stone. The stone was the final piece in the ritual to open a portal to Mankar Camoran's Paradise, where he waited with the Amulet of Kings.

Background
In the later days of the Oblivion Crisis, things were complicated and essentially put into a halt by Gareth's rise to power. By creating a false Amulet of Kings and relighting the Dragonfires, he had effectively stopped the daedric invasion. Unfortunately, the false Emperor also proved to be a tyrant who attempted to enslave all of Tamriel's citizens much like his Ayleid ancestors had done so before.

Before Gareth took power, the Hero of Kvatch had nearly obtained all of the items needed to open a portal to Mankar Camoran's Paradise realm, which would allow him to challenge Camoran for the real Amulet of Kings so that Martin Septim could claim the Imperial Throne and stop the invasion. Before this could happen, however, the events of the Siege of Castle Relleis occurred in which Gareth created a portal to the past to alter time and become Emperor. Nearly two years later, Ra'rgo, who was also sent into the portal, returned to the present day (now considered the second year of the fourth era) in order to regroup with the Knights of the Nine and stop Gareth. In the process, however, he managed to darken the Dragonfires once again, restarting the Oblivion Crisis.

Martin Septim and his Blades guard (who had retreated silently to Skyrim after a siege of Cloud Ruler Temple by Gareth's forces) returned to Cyrodiil to reunite with Ra'rgo, who learns of Martin's plan to finish the ritual: they need a Great Sigil Stone, which can only be obtained by entering a Great Gate like the one that led to the destruction of Kvatch. The only way to open another one is to bait the Mythic Dawn into doing so, which he believes can be done by making the city of Bruma a target too good to turn down.

Before the events of Gareth's rise to power, Ra'rgo had assembled an army of volunteers from most of Cyrodiil's cities which had been camped out in Bruma for quite some time. Most of the army disassembled on the order of the new Emperor, but upon Ra'rgo's return many of them received word and moved back to Bruma. Moreover, Ra'rgo ordered the bulk of his Knights of the Nine - roughly a hundred knights - to reinforce the city's garrison.