If a unit dies to Challenge, can you play Immortal Phoenix from trash (does Challenge count as a spell killing the unit)?

Ruling: You would not get the trigger for Immortal Phoenix when a unit dies to Challenge.

Nuances:

• The unit dealt the damage, not the spell directly, even though the spell instructed the damage action
• For Unyielding Spirit's purposes, this is not spell damage, but there was debate about whether the spell has "kill action responsibility"
• The same question applies to Void Gate
• An old rules FAQ addressed this but has been officially retired, so its guidance can only suggest intention