The hero plays as a death-like being. They must slay these humanoid monsters in purgatory to send them on their way and collect their soul (orbs?). Either after every X amount of souls collected or when death "dies" - they see the bone man. If they meet his quota he offers his "reward" by choosing one of three choices that will improve the hero but also punish the hero. E.g the hero can select "gain 15% more souls per kill but monsters increase respawn time". The idea is the hero is punished to improve their power.
If the hero dies before meeting the bone mans quota - he does not give you a reward but instead gives you a random punishment and then sends the hero on their way back to the arena starting at the current amount of souls they died with to allow them to reach the reward a bit faster.
If the hero fails to the meet the bone mans quota after 3 times the bone man will fire the death-like being and the player loses and must restart.
If the hero reaches a certain amount of souls to buy the final choice (the 4th ranked choice on every reward) this will end the game in some sort of fashion. How that happens is TBD.