When Glass Mixologist is being attacked and dies in combat, while I have Galio in my trash, can the Glass Mixologist deathknell trigger and bring back the Galio onto the battlefield? Explain in detail the interaction.

Yes, you can use Glasc Mixologist's Deathknell ability to play Galio, Indefatigable from your trash to the battlefield where the combat is occurring.

The Interaction

According to Rule 808.1.d.2, Deathknell triggers are added to the chain as a pending item before the unit is moved to the trash. Because the game preserves the board state during combat, you are permitted to resolve the Deathknell before the battlefield is officially lost or the combat cleanup concludes.

Sequence of Events

1. Combat Damage Step: Combat damage is 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 State: Because an item is on the chain, the game state becomes CLOSED.
4. Chain Resolution: You resolve the Deathknell effect. You may play a unit from your trash that meets the ability’s energy cost requirements.
5. Galio Enters: Galio, Indefatigable is a valid target for the effect. You play him from your trash to the battlefield.
6. Combat Resolution: After the chain empties and the Deathknell resolves, the combat continues. Since a new unit (Galio) is now present, you maintain control of the battlefield and prevent the opponent from conquering it.

Key Details

• Combat Continuity: Because you are the defender, you maintain control of the battlefield throughout the combat. Playing Galio via Deathknell allows you to keep a unit on the board, which fulfills the requirement to maintain control and prevents the opponent from successfully conquering the battlefield (as established in RiftJudge FAQ #9149).
• Galio's Role: Once on the battlefield, Galio will act as a "meat shield" for the remainder of the showdown due to his [Tank] keyword, even though he deals 0 combat damage.

Note: I could not fully verify this ruling against the retrieved rules, as certain card-specific stats and symbol interpretations could not be confirmed.