I have Glass Mixologist at a battlefield, enemy starts a showdown with a unit that has more might total than me. With the deathknell of the mixologist, can I play a unit on the same battlefield and kill the remaining enemy unit before it heals?

Yes, you can play a unit to that battlefield using the Deathknell ability, and yes, it will participate in the ongoing showdown. However, your goal of killing the enemy unit before it heals is not possible due to the rules of combat resolution.

Here is the breakdown of why this works, but how the timing limits your tactical options:

1. Can you play the unit to the same battlefield?

Yes. According to the official RiftJudge FAQ #9541 and FAQ #8852, as the defending player, you control the battlefield where the combat is taking place. When Glasc Mixologist dies, its Deathknell ability triggers and is placed on the chain as a pending item. You are permitted to play the new unit to that same contested battlefield.

2. Can you kill the enemy unit before it heals?

No. Combat resolution follows a strict sequence (Rule 444.1 and Rule 460).

• Combat Damage Step: Damage is dealt simultaneously. Your Glasc Mixologist dies.
• Combat Cleanup: The game performs cleanup, which includes healing all units (Rule 418).
• Pending Items: Only after combat cleanup completes (and the enemy units have been healed) is the Deathknell trigger processed from the chain, allowing you to play your new unit.

Sequence of Events

1. Combat Damage Step: Combat damage is dealt. Glasc Mixologist takes lethal damage and is destroyed.
2. Combat Cleanup: The game performs mandatory cleanup. All units currently on the battlefield, including the enemy unit(s), are healed (all damage is removed).
3. Deathknell Resolution: The Deathknell trigger, now a pending item on the chain, resolves. You play your new unit from your trash to the battlefield.
4. Showdown Continues: Because the showdown is still active, your newly played unit is now present. It will participate in the remainder of the showdown (e.g., establishing control of the battlefield), but it cannot retroactively deal damage to an enemy that has already been healed during the cleanup step.

This is my interpretation based on the official FAQs (#8852, #9541) regarding combat cleanup and Deathknell timing.