Does Unyielding Spirit prevent damage from Snapvine's ability that makes two units deal damage equal to their mights to each other?

Ruling: Unyielding Spirit does not prevent Snapvine's damage because the units themselves are the source of the damage, not the ability.

Nuances:

• The key distinction is whether an ability says "deal damage" (ability is the source, blocked by Unyielding) versus "[unit] deals damage" or "they deal damage" (unit is the source, not blocked by Unyielding)
• This same principle applies to Challenge and Last Breath, which also bypass Unyielding Spirit
• Abilities like Yasuo and Caitlyn that say "deal damage" (without specifying the unit as the dealer) are blocked by Unyielding Spirit because the ability itself is the damage source