If my Glasc Mixologist dies when the opponent conquers my battlefield, am I able to play a unit to that battlefield after it is conquered via Glasc Mixologist deathknell?

Yes, you can play a unit from your Deathknell ability to the battlefield, provided the ability resolves before the battlefield is officially conquered.

According to the rules and FAQs governing Deathknell and battlefield control, your ability to play a unit depends on the timing of the Deathknell trigger relative to the conquest.

The Ruling

While Deathknell triggers are added to the chain as pending items before a unit is moved to the trash (Rule 734.1.d.2 / 323.4), Deathknell abilities resolve before a player establishes control and conquers a battlefield. As the defender, you maintain control of the battlefield while the Deathknell is on the chain. Because the resolution of the Deathknell effect occurs before the conclusion of combat resolution, you have the permission to play a unit there, which can prevent the opponent from successfully conquering the battlefield.

Sequence of Events

1. Combat Damage: Damage is dealt; the unit takes lethal damage and dies.
2. Deathknell Trigger: The ability triggers and is added to the chain as a pending item.
3. Deathknell Resolution: The Deathknell resolves. You play a unit from your trash to the battlefield, effectively preventing the opponent from successfully conquering it.
4. Combat Resolution (Control Shift): The combat resolution steps (Rule 461) process. Since the unit was played via Deathknell, the requirements for conquest are no longer met.

Key Clarification

• Ongoing Combat: The Deathknell triggers during combat cleanup (Rule 323.4). Because control of a battlefield cannot be lost while items are on the chain (Rule 187.4.c), you can resolve the Deathknell to play a unit to that battlefield, preventing the opponent from successfully conquering it.
• Conquered Battlefields: Once the battlefield has already been conquered, the combat resolution is complete, and control has shifted. You cannot play a unit to an opponent's battlefield via Deathknell after control has been lost.

This interpretation is based on the interaction between Rule 187.4.c (control during chain items) and the established rulings in RiftJudge FAQ #7489, #9149, #10335, and #10444, which clarify that Deathknell resolves prior to the establishment of control and final conquest.