Can I react to the Deathknell trigger of Unsung Hero to reduce its Might in order to prevent the card draw?
No, you cannot prevent the card draw by reducing Unsung Hero's Might in response to its Deathknell trigger.
According to RiftJudge FAQ #4157 and Rule 808.1.d.3, the Mighty condition is checked when the Deathknell resolves, but it uses the attributes and details noted at the moment of death.
Why this interaction works this way:
- Snapshotting: When a unit with Deathknell dies, the game notes its location, attributes (including its Might), and other relevant details before it is moved to the trash (Rule 808.1.d.3).
- Triggered Ability Resolution: Even though you can place a reaction on the chain in response to the Deathknell trigger being placed, that reaction will not change the snapshot taken at the moment of death. The Mighty check for Unsung Hero looks at the "noted" Might value from the moment the trigger was created, not the unit's current Might (which is technically null or non-existent since it is already in the trash).
Therefore, if Unsung Hero had 5 or more Might at the exact moment it died, the Deathknell effect will successfully trigger the draw, regardless of any attempts to reduce its Might after the trigger is already on the chain.