Can you play units summoned from Glasc Mixologist to the battlefield that it died at?

Yes, you can play a unit summoned from Glasc Mixologist's Deathknell ability to the battlefield where it died, provided you control that battlefield.

According to the RiftJudge FAQs regarding Glasc Mixologist (specifically FAQ #9149, #9329, and #8852), the Deathknell trigger is placed on the chain as a pending item during the cleanup process (Rule 322.3 and Rule 734.1.d.2). Because the game pauses to resolve this chain before finalizing the cleanup and control checks, you are permitted to play a unit to that battlefield. If you successfully play a unit there, you will still have a unit present when the game checks for control, allowing you to maintain ownership of the battlefield.

Important Distinction: Attacker vs. Defender

Whether you can play the unit depends on your status as the controller of the battlefield:

• If you are the Defender: You already control the battlefield where the combat is taking place. You are permitted to play the unit there via Deathknell (FAQ #9149, #8852).
• If you are the Attacker: You do not control the battlefield until the showdown is resolved and you have successfully conquered it. Therefore, you cannot play a unit to that battlefield during the combat cleanup phase (FAQ #9479, #9314).

Sequence of Events (as a Defender)

1. Combat Damage: Damage is assigned and dealt; Glasc Mixologist takes lethal damage and dies.
2. Deathknell Trigger: Before Glasc Mixologist is moved to the trash, its Deathknell ability triggers and is added to the chain as a pending item.
3. Chain Resolution: The game pauses to resolve the chain. You resolve the Deathknell effect and play a unit from your trash to the battlefield.
4. Control Check: After the chain resolves, the game checks for control of the battlefield. Because you played a unit there, you maintain control.

Note: This is based on the provided RiftJudge FAQs. No official rule chunk explicitly details the "simultaneous death" interaction, but it is consistent with the established Riftbound FAQ rulings.