The Faction Pack is made up of three side missions dealing with individuals who don't have anything to do with Artyom's story. That being said, their roles in this world cannot be overstated; at several points over both of the games, Artyom crosses paths with the Nazi Reich, the Communist Red Line, and the Rangers of Polis. The Faction Pack gives you the opportunity to step into the shoes of a soldier from each faction.
Heavy Squad, the Nazi Reich mission, is the weakest of these three. You are cast as Reich shock trooper Hans during the climactic Battle of D6. What's the objective? Well, you can't really go anywhere, so all you do is hold the line while killing as many encroaching Red Line soldiers as humanly possible. Metro: Last Light's straightforward gunplay isn't great, and it's best used in stealth-action hybrid encounters. Heavy Squad is a difficult and short mission that won't stay with anyone who plays through it to the end.
Kshatriya is perhaps the best part of the Faction Pack. In it, you play as a Polis Ranger Trainee, a Stalker charged with the task of venturing to the inhospitable surface and collecting as many preapocalypse artifacts as possible from the incredibly perilous Great Library. You will return to your underground station often to purchase new equipment, and also to save, as there are no autosaves in this mission. As you work your way deeper into the Library, you find special doors that can only be unlocked from the Library; these lead directly back to the station and give you a series of convenient shortcuts that prevent you from having to retread old ground countless times. This sense of forward progress has a distinct RPG feel to it, and I'd like to see more of this in future games.
Sniper Team casts you as a nameless Red Line soldier on a mission to infiltrate Black Station, a heavily-guarded stronghold under the control of the Nazi Reich. Black Station is not only plagued with Nazis, but it's considerably more irradiated than the rest of the surface. Certain areas cause your filter timer to run down at an alarming rate, and you'll need to be extra vigilant in your search for additional filters. Oh, and if you're spotted, the mission is automatically failed. Good luck!