If my Glasc Mixologist dies during combat from an opponent's attack and I use its Deathknell to play Xin Zhao to that battlefield, can I do so and what happens next?

Ruling: Yes, you can play Xin Zhao to the battlefield where combat is occurring. As the defender, you retain control of the battlefield throughout combat until the Establish Control step, which allows you to play units there via the Deathknell trigger. After the current combat concludes, all units are fully healed and a new combat is staged between Xin Zhao and the attacking unit.

Sequence:

• Glasc Mixologist dies during combat and Deathknell triggers
• Play Xin Zhao from trash to the battlefield (you control it as defender)
• Current combat concludes and enters Resolution Step
• All units at the battlefield are fully healed
• Combat result is determined as "No Result" (both players have units present)
• A new combat is staged between Xin Zhao and the opponent's unit

Nuances:

• Control of the battlefield, not participation in combat, is what determines whether you can play units there
• The defender retains control throughout combat until the Establish Control step
• Xin Zhao enters ready (not exhausted) unless his ability conditions aren't met