Shadow of the Demon Lord uses a d20 to resolve an action when the GM is uncertain about the outcome. Like many games that use a d20, SotDL has you roll a d20 and add the modifier from the attribute that applies best. The target number is fixed at 10 if the action is not a task or the score of the attribute or characteristic used to resist the attack.
For easier and harder tasks, the game uses boons and banes. For each positive circumstance that would help you with the action, you have a boon. For each negative, you have a bane. Boons and banes cancel each other out so you only have one at a time. For each boon/bane, you roll a d6. You add the highest boon rolled to your roll of a d20. You subtract the highest bane rolled from your roll of a d20.
This does some useful things. One, pinning the target number at 10 or score makes it easier to adjudicate what happens in the game. Two, boons and banes can be added freely beyond the first as the highest number you will add or subtract is 6.
For more information, see: http://schwalbentertainment.com/2015/02/18/evolving-design/