If I use Undying Spirit in response to a damage spell, can the opponent chain another reaction damage spell that will still deal damage before Undying Spirit resolves?

Ruling: Yes, an opponent can cast a reaction spell in response to Undying Spirit, which will resolve first due to chain priority and deal damage before Undying Spirit's protection takes effect. Once Undying Spirit resolves, units can still be targeted by damaging spells for the rest of the turn, but all damage will be prevented.

Sequence:

• Opponent casts damage spell (e.g., Icathian Rain)
• You respond with Undying Spirit
• Opponent can respond with another reaction damage spell
• Opponent's reaction spell resolves first and deals damage
• Undying Spirit resolves, preventing all further spell damage until end of turn

Nuances:

• After Undying Spirit resolves, units can still be targeted by damage spells, but the damage is prevented
• The protection lasts until the end of the turn