When the Underworld Bonus Game starts, you will notice there are multipliers sitting underneath each reel. These get activated when the Green Soul Orb lands on that reel. The potential multipliers range from 2x, 3x, 4x, 5x, and 10x. Also, each time a new multiplier gets activated, 3 extra spins are handed out. As you progress, the value of all the activated multipliers gets totaled and multiplied by the overall value of each cluster.