Can you respond to Reckoner's Arena trigger by killing a unit (like Qiyana) to prevent its conquer effect?

Ruling: Yes, you can respond to the Arena trigger by killing a unit to prevent its conquer effect from being added to the chain.

Sequence:

• Arena's trigger goes on the chain
• You can respond by killing the unit before the trigger resolves
• When Arena's trigger resolves, it checks which units are present at the battlefield with conquer abilities
• Only units present when the trigger resolves are added to the chain
• If a unit was killed in response, it will not be included