The Sacrifice

The Sacrifice is a three-chapter DLC for both Left 4 Dead and Left 4 Dead 2, it was released on October 5th, 2010.

The Sacrifice takes place after Blood Harvest, and is considered to be the prologue to The Passing, as both campaigns are connected to each other. The DLC is free for PC players, while for Xbox 360 players, it comes in two separate packages on Xbox Live (one for Left 4 Dead and one for Left 4 Dead 2), and costs 560 Microsoft Points for each one.

The campaign's name comes with the obvious interpretation that one of the Survivors has to sacrifice themselves in order to enable the survival of the team. Canonically, Bill is the one who makes this choice, as seen by his unfortunate appearance in The Passing's finale inside a generator room, as well as the comic. The tagline, "It's your funeral," refers to the idiom "it's your funeral", basically meaning someone may have made a bad choice, and if they follow up on it, they will have to endure the direct consequences regarding that choice. Should the survivors decide to sacrifice nobody, a continual horde and endless amount of tanks will spawn until they eventually overrun the survivors.

Walkthrough

 * 1) The Docks
 * 2) The Barge
 * 3) Port Finale

Both Left 4 Dead and Left 4 Dead 2

 * The Sacrifice campaign.
 * New dialogue for all the original Survivors, including community ones for Bill.
 * A new item "Explosive Barrel," which is an unmovable combination of the gas can and propane tank.

Only for Left 4 Dead 2

 * A port of No Mercy which includes usage of the Left 4 Dead Survivors.
 * Five New Mutations: Taaannnkk!!, Hunting Party, Lone Gunman, Bleed Out Versus and Healing Gnome (only for users who have bought or own The Passing DLC).
 * New dialogue about the Left 4 Dead 2 Special Infected for Bill.
 * The CEDA Worker Infected and the Worker Infected also appear in this campaign and in No Mercy.

Achievements
Unlike other campaigns, The Sacrifice's Achievements can be obtained separately on both Left 4 Dead and Left 4 Dead 2. Two of the Achievements, SUPREME SACRIFICE and KILL BILL, are only obtained by the player who sacrifices themselves in Left 4 Dead, while in Left 4 Dead 2, it counts for the whole team.