Deadly Crossbow is a missable crossbow found only in Chapter 19. It is green in color unlike the other crossbows. To obtain it, Zael must pause in the room Horace falls in. Look up, grab your crossbow, and use Burst Arrows to shoot the crystals above the chest. Watch it fall down upon the chest, and into the hole. Then when you agree to enter the hole, grab the chest and move on.
The Deadly Crossbow has an 80% chance to inflict Doom on headshot. It also has a 30% chance to deal 8x damage.