If Ekko dies in combat and readies runes with its Deathknell ability, can you use those runes to play an action spell or Shen before the enemy unit heals or scores?

Ruling: You cannot play an action spell after Ekko dies but before the enemy scores - there is no window for this. You can only play reactions if abilities trigger during combat resolution (like Conquer abilities).

Sequence:

• Combat damage is applied and Ekko dies
• Ekko's Deathknell triggers and readies runes
• If Conquer abilities trigger, you can play reactions before combat ends
• Enemy unit heals (if applicable)
• Enemy scores (if applicable)

Nuances:

• You cannot play Shen using the runes from Ekko's Deathknell if Ekko was your only unit on that battlefield, because you lose control of the battlefield immediately when you have no units there, and you can only play units to battlefields you control or to your base
• If you somehow still had another unit on the battlefield when Ekko died, you could play Shen in response to the Deathknell trigger (before scoring) or Conquer trigger (after scoring), but Shen wouldn't apply damage since it wasn't there when combat damage was resolved
• If the opponent conquers the battlefield, you lose control and cannot play Shen there to prevent the conquest