Challenge

If a spell or ability instructs a particular object to deal damage, including the card the ability is on, then that object is dealing the damage, not the spell or ability. So Unyielding Spirit does stop damage from Void Seeker, Stormbringer, or Yasuo, Remorseless, but does not stop damage from Challenge, Last Breath, or Carnivorous Snapvine.

- You play a spell like Challenge that causes a unit to deal lethal damage to another unit.

417.6.b.3. When a spell or ability specifies a Unit as the source of the Damage for the Deal action, it is not in addition to the spell or ability that instructed it. Example: Challenge is a spell that reads "Choose a friendly unit and an enemy unit. They deal damage equal to their Mights to each other." The damage that Challenge causes to be dealt is dealt by the chosen units, not by Challenge.

417.6.b.4. The controller of the source of a Deal action is responsible for that Deal action unless the player performing the Deal action is otherwise specified. Example: If a player plays Challenge targeting a friendly unit and an enemy unit, the controller of the enemy unit is responsible for the damage dealt by their unit. Any effects that trigger "when you deal damage" that that player controls will trigger.