Defeat of Legate Lanius at Zion Canyon

In late May 2282, Caesar is assassinated in New Vegas, leaving Legate Lanius in charge of the Legion. Being a single-minded brute, he turns his sights towards the only thing he knows--revenge. He swears vengeance on Joshua Graham and the Latter Day Nephites, blaming them for Caesar's death, and announces a campaign north to enact it. However, before he can begin this effort, the praetorians led by Lucius and what remains of the frumentarii come together to challenge his authority, claiming Lucius's seniority as making him the rightful heir to the Legion. The conflict continues until September, when Lanius emerges victorious, all the praetorians slain and a good portion of the frumentarii killed, others fleeing and the rest using their skills to disappear into the other ranks of the Legion.

Following this, Lanius begins the long and difficult process of consolidating his authority and resources in an increasingly fragile Legion, pursuing his singular goal with no real long-term plans other than vague talk of conquest and war. By early April 2283, his forces begin the march to Zion Canyon, arriving there in early May 2283. There Lanius faces a disastrous defeat at the battle of the Twin Legates, his death heralding the final collapse of the Legion.

Despite his overwhelming numbers, a complicated series of factors cumulates in his devastating loss at the hands of Joshua Graham, Jessica Lexington, the Latter Day Nephites, and the Sorrows and Dead Horse tribals, who came together and brilliantly strategized a victory.

Lanius's strategic failures
As terrifying as he was as the Monster of the East, Lanius did not fare well as a strategist, lacking Caesar's mind for logistics and vision for the future. On multiple levels he set himself up for failure, the first being his short-sighted adherence to his honor code and the repercussions of it, the next his poor ability to maintain order and oversee the logistical needs of an army, and the last his inability to combat guerrilla tactics in unfamiliar terrain.

Dissent in his ranks and the dissolution of the frumentarii
When Vulpes Inculta was supposedly slain in New Vegas, the frumentarii were left leaderless and confused, but saw the way the wind blew in the Legate's open hatred of their tactics and operations. Meanwhile, Lucius came to question Lanius's leadership when their number one priority was turned towards a fool's errand in the north, rather than consolidating and securing their new territory in the mojave, and ignoring the perfect opportunity to defeat and seize the faltering NCR. When the time came to organize a coup, Lucius's praetorians were more loyal to him than to their Legate, and the remaining hidden frumentarii quickly smelled the dissent, pledging themselves to Lucius in return for the re-establishment of their positions.

With the subtlety of the frumentarii and the strength and respect the praetorians garnered, a non-insignificant portion of the Legion sides with the rebels, starting a months-long struggle that continues from May to September, involving complicated frumentarii machinations to quietly win over the loyalty of important centurions to ensure that the Legion wouldn't unravel at the death of Lanius, then using the praetorians to assassinate key Legionaries who supported Lanius. With no expertise at all in stealth, Lanius was unable to root out the Legionaries protected by the frumentarii, finally ending the conflict by simply killing anyone who wasn't explicitly loyal to him, taking out many centurions and decanii and a significant portion of the legionaries that served under them, culling the numbers of Legion in the mojave. Soldiers who weren't prepared for promotions found themselves suddenly in positions of leadership, making the ensuing process of organizing Legion authority and governance a difficult and drawn-out task.

It took until April of 2283 before Lanius had the Legion in a stable enough condition to begin his march north to Zion Canyon. Gathering the weapons, supply stockpiles, and maintaining and organizing supply caravans for the march could be described as rocky at best. With the frumentarii gone and most of the best centurions and decanii killed, inefficiencies and various shortages wracked the mojave's Legion presence, their reduced numbers not helping keep order.

Infiltration by Resistance agents and mass-mutiny at the Canyon
The main Resistance was still very much active after the death of Caesar, helping stoke the conflict within the Legion's own ranks and helping to disrupt supply lines and troop deployments into the mojave. However, the Latter Day Nephites took a more active approach to their anti-Legion operations. With resources to organize in the north, intelligence was the name of the game. Nephite agents kept close tabs on the events in the mojave, sending reports back to Zion Canyon, Dead Horse point, and eventually the group of settlers and church officials (including Byrum Pratt) in the remains of Salt Lake City who'd come to form the city of New Zion later.

With legitimacy of rule being the primary factor driving inter-Legion conflict, the Nephites began to concoct a plan to further divide Legion ranks. It's unsure if the Nephites themselves came up with the rumor or if they were playing up an existing one, but as early as late 2282 whispers began to be passed around that the Malpais Legate wrenched himself out of Hades itself to reclaim the Legion now that Caesar was dead. With internal conflict in the Legion, questions of legitimacy were at the forefront of the minds of many Legionaries, and with the surviving frumentarii extremely bitter at their leadership, they were willing to work with Nephite spies to help sow these rumors and myths.

First defection
By January of 2283, the first defections had begun, with small scattered groups breaking off and heading north to find the Malpais Legate. The first group (including Felix Octavius), was led by several frumentarii who had gathered/persuaded some other legionaries for protection, and arrive at the canyon to the great surprise of Lexington and Graham. Lex immediately picks up on what's happening, and in a knee-jerk attempt to assert authority over the legionaries she immediately regrets, she proclaims that she, having slain the last Vulpes Inculta, has inherited the title in the ways of Legion tradition, and that she has been orchestrating the Nephite agents who were encouraging the defections (she wasn't, but she guesses now she fucking is LMAO). So that's a very fun conversation she has with Graham immediately after, explaining what apparently is happening. He's naturally mortified at the scenario, but after some convincing Lex persuades him to go along with the act, Graham unable to deny the massive opportunity of causing widespread defections.

Having seen the Malpais Legate themselves, some of the frumentarii return to New Vegas to confirm the rumors, sparking a new trickle of defections that slowly come into the canyon, helping to add to the war camp's numbers. Lanius is oblivious to these efforts and with mostly unprepared leadership forming his ranks, the frumentarii easily conceal their schemes.

Mass defection at Zion Canyon
With the trickle established, planned is then a much more ambitious surprise defection. Spread amongst those susceptible is a scheme that will unfold at Zion Canyon--when the Dead Horse war horn is blown, those involved in the plan will run to high ground and attack their comrades below. Part of the strategy to fight the Legion at the canyon also involves damming the Virgin river and unleashing a flood, which high ground will protect the defectors from. When the time comes at the actual battle, the plan is put into action and a good 25% of Lanius's forces turn on him, with about half of the others picked off by the flood, leaving a much smaller number to combat. Part of this mass defection also involves the frumentarii having quietly organized dissent amongst their supply lines too, so when the battle is over, they still get support coming in for a significant period of time, so the defectors are supplied and fed.

Lanius's pride and the Emerald Pools trap
Legate Lanius is, at the very least, quite consistent in his honor code, which becomes a greatly exploitable fact at Zion Canyon.