Can you counter Challenge with Unyielding Spirit?

Ruling: No, Unyielding Spirit does not stop damage from Challenge.

Nuances:

• Unyielding Spirit does stop damage from units like Void Seeker, Stormbringer, or Yasuo, Remorseless
• Unyielding Spirit does not stop damage from Challenge, Last Breath, or Carnivorous Snapvine
• The "they deal damage to each other" wording in Challenge does not count as spell damage that Unyielding Spirit can prevent