Souls are ancient artifacts in possession of each boss. You will receive 4 random souls each time you defeat a boss. There is also one extra spot for a hidden soul to be revealed which costs 20 gold. Each player has the choice to spend that gold for the benefit of his/her team and drop one extra random soul in the shop.
At the start of each resting period between boss fights, souls are up for auction which gives you 20 seconds to choose the souls that you'd like to buy. Everyone gets the chance to bet their karma on the soul that they would like to buy. At the end of the 20 seconds, there is a random roll that rolls a number for each person who has chosen to buy each soul. The person with the highest numbers lose the karma they have spent and get to keep the soul while the rest of the players will get their karma back and have a chance to buy whatever soul that's left in the shop.