If a Glasc Mixologist dies at a battlefield can the unit it revives be played at the same battlefield?

Yes, if Glasc Mixologist dies at a battlefield you control, the unit you revive with its Deathknell ability can be played to that same battlefield.

Why this works

According to the Unleashed rules update (specifically the clarification regarding Rule 187.4.c), control of a battlefield cannot be lost while there are items on the chain. When Glasc Mixologist dies, its Deathknell ability is placed on the chain as a pending item before the unit is moved to the trash and before the game performs the cleanup that would typically result in you losing control of an unoccupied battlefield.

Because the game pauses to resolve the Deathknell effect on the chain, you are permitted to play your revived unit to that battlefield. Since you still have a unit present at the battlefield when the cleanup finally finishes and control is checked, you successfully maintain control.

Important Distinction: Attacker vs. Defender

Your ability to play the unit to the battlefield depends on your status at that location:

• If you are the Defender: You already control the battlefield. You are permitted to play the unit there to maintain your control.
• If you are the Attacker: You do not control the battlefield until the showdown is resolved and you have successfully conquered it. Because you do not control the battlefield, you cannot play a unit there during the combat cleanup phase.

Sequence of Events (as Defender)

1. Combat Damage: Damage is dealt; Glasc Mixologist takes lethal damage and dies.
2. Deathknell Trigger: The Deathknell ability is added to the chain as a pending item before the unit is moved to the trash (Rule 734.1.d.2).
3. Chain Resolution: The game pauses to resolve the chain. You resolve the Deathknell effect and play your unit from your trash to the battlefield.
4. Control Check: Because the unit you played is now present, the game confirms you still control the battlefield during the cleanup process.

This is based on the rules for battlefield control and Deathknell timing as detailed in the Riftbound Unleashed rules update.