Cubile Animus (Latin for "Nest of Souls") is an ancient box buried in the cemetery ground that can trap and contain souls. Jesse, aka Reverand Black, wanted to find the box so he could use it to revive the Black Army, his 219 followers that were burned alive. He planned to do this by taking the souls from the 219 Duskers at the advance screening of Dusk III: Unbitten and transfer them to revive the Black Army. Ethan, Benny, and Sarah stopped him by opening the box with the souls inside and letting the souls fly straight through Jesse's heart, killing him.
Jesse was revived when Ethan, Benny and Rory dug up the box to trap a demon that had taken possession of Sarah's body and dropped it to the ground while digging. Also, the night of the Duskpreview, Jesse had Erica on the vampire side, as always. She would give anything to turn on Sarah.