How does damage sourcing work in Riftbound, specifically why does Challenge damage not get blocked by Unyielding Spirit?

Ruling: Challenge damage is not blocked by Unyielding Spirit because Challenge specifies that the chosen units are the source of the damage, not the spell itself. When a spell or ability specifies a unit as the damage source, it replaces the spell/ability as the source rather than being in addition to it.

Sequence:

• When Challenge is played, it specifies that the chosen units deal damage to each other
• The damage source is the units themselves, not the Challenge spell
• Therefore the damage is not spell damage and bypasses Unyielding Spirit

Nuances:

• When an ability deals damage without specifying a unit as the source (like Hextech Ray), the spell itself is the damage source
• When an ability is the source of damage, it is in addition to the spell or unit that created that ability (like Iron Ballista dealing damage as both a gear and an ability)
• Yasuo, Remorseful's damage would be blocked by Unyielding Spirit (ability is additional source)
• Snapvine's damage would go through Unyielding Spirit (unit is specified as source)