Can players play reactions after damage has been assigned to a specific unit during combat?

Ruling: No, it is not possible to play spells (including reactions) during combat resolution/the combat damage step. The next opportunity to play spells would be in response to conquer or deathknell triggers, but by that point surviving units have already healed.

Sequence:

• Showdown phase is when players can play reactions and other spells
• Combat damage step occurs (this is mainly for calculating damage)
• No spell window exists during combat damage assignment
• Next window opens in response to conquer or deathknell triggers
• Surviving units have healed by this point

Nuances:

• The showdown phase is the intended time to play reactions affecting combat
• The combat damage step is primarily for mathematical resolution, not for playing spells