Mod:Fallout: The Frontier/The Bridge on the River Willamite

I Am Legion is the first Legion quest, granted upon asking to join the Legion during A Bridge Too Far.

Quick walkthrough

 * Retrieve your items
 * Talk to Callidus
 * Head to Outpost Henderson and deal with the NCR there, through combat or persuasion
 * Fight a mysterious robot or shut it down with a Science 90 check
 * Talk to the messenger and return to Legate Valerius

Detailed walkthrough
Having completed I Am Legion, Legate Valerius will tell you to report to Callidus for further instructions. While in Valerius's office, retrieve your items from the "Item Box" crate under his side-desk.

Callidus will ask you to head to Outpost Henderson and deal with the NCR stationed there. There are three NCR Soldiers and one ranger there, and the simplest solution is to simply kill them. Alternatively, you can talk to the ranger, Ian Malcolm, and convince them to leave provided you have any one of:
 * Speech 80
 * Repair 60
 * The Terrifying Presence perk

After you're done, exit the outpost and you'll be confronted by a mysterious robot. The robot will inevitably turn hostile unless you pass a Science 90 check and automatically shut it down, skipping the fight. Once the robot's down, a messenger will show up and ask you to report back to the Legate; return to Ostia and do so to complete the quest.

Rewards

 * 650 XP
 * Positive Northern Legion reputation