After all four Amplifiers are destroyed, a purple Lantern will appear, floating from the Courtroom to the Church or from the Courtroom to the Jail. The player needs to use a grenade to knock the Lantern out of the air, timing it such way that the grenade explodes when it is right next to the Lantern.
The Lantern will fall down on the ground and the player has to pick it up by interacting with it.
Subsequently, the player has to charge the Lantern with the life force of the zombies. However, killing zombies directly won't work so you have to use the Trample Steam, the Subsurface Resonator, the Head Chopper, a Nuke Power-Up or feed the Big Guy a candy so he will kill zombies around you.
Maxis will say that the "energy within the Lantern is essential for exposing spirits of those who came before you". With this he means that once the Lantern is fully charged up, you have to place it on the roof of the Gunsmith Shop