The Undead Nightmare Pack Add-on for Red Dead Redemption has 12 achievements worth 200 gamerscore.
Complete "On a Pale Horse".
Complete "Mother Superior Blues" Survivor Mission.
Have every territory saved at the same time during the Undead Nightmare.
Complete "Cure For Most of What Ails You" and "Get Back in that Hole, Partner" Survivor Missions.
Find and kill a sasquatch.
Complete "Curious Tales from Blackwater, USA" Survivor Mission.
Attain 100% Game Completion statistic in Undead Nightmare.
Attain Rank 5 in all Undead Nightmare Challenges.
Find and break a unicorn.
Find and kill a chupacabra.
Make it to wave 15 in Undead Overrun Gametype in Multiplayer.
Fight off 8 unique players during your time on top during Land Grab in Multiplayer Free Roam.