Throughout the first 10 levels of the game, you will find several notes from a deceased prisoner called Toorum. Find all 16 (before advancing to Level 11) then locate Toorum's remains on Level 10. Use his remains on the Crystal of Life to revive him, unlocking the "Buddies With Toorum" achievement. This also allows you to play as him in a solo adventure on a new playthrough (see "Play as Toorum").