Mod:Fallout: The Frontier/Nuclear Winter

Nuclear Winter is an optional main quest

Background
If you opt to kill Legate Valerius during a Legion playthrough, you will receive the option to permanently leave the frontier, akin to the Honest Hearts quest Chaos in Zion.

Quick Walkthrough

 * Find a map to the Frontier on Marcus's person.
 * Find Vertibird parts on Elias Blackthorne. Note that the NCR Exiles will be hostile.
 * Take the Vertibird back to the Mojave.