In a commander game I attack the player to my left. He puts Eye for an Eye on the stack and then the player to his left puts Darkness on the stack. They both resolve. Who takes damage if any when we get to combat damage step? Would it be different if the defending player had cast both of those spells?
You'll take some damage from Eye for an Eye, maybe. It depends on how vindictive the player you're attacking is feeling. Both Darkness and Eye for an Eye want to modify/prevent the damage being dealt, so the player that would be dealt damage gets to choose which effect to apply to the combat damage from your creature first. If your opponent chooses to apply Darkness's effect first, then your creature won't deal any combat damage this turn and Eye for an Eye's effect will no longer apply to that damage. If your opponent applies Eye for an Eye's effect first, then the combat damage to your opponent will be replaced with that same amount of combat damage and that amount of damage to you from Eye for an Eye. Darkness's effect is then applied and prevents the damage to your opponent, but you'll still be dealt the damage from Eye for an Eye.