If a defending unit with Deathknell (like Glasc Mixologist) dies in combat and its Deathknell ability plays a new unit to the battlefield, does a new combat start between the attackers and the newly played unit?

Ruling: No. The Deathknell trigger does not resolve until after the combat has completely ended, so the newly played unit enters the battlefield after combat resolution is complete and does not trigger a new combat.

Sequence:

• Combat damage is dealt; the defending unit with Deathknell takes lethal damage
• FEPR is skipped (Rule 460.3), so the Deathknell trigger becomes a pending item that must wait
• Combat Cleanup occurs: the unit dies and all surviving units are healed (Rule 461.1.a.1)
• The Resolution Step completes all outstanding tasks: Determine Combat Result, Establish Control, and Combat Ends (Rules 461.3-461.7)
• Only after the combat fully ends does FEPR resume and the Deathknell trigger resolve, playing the chosen unit to the battlefield
• Because the new unit was not present during the "Determine Combat Result" step, it does not cause a "No Result" scenario and no new combat is staged

Nuances:

• Units are healed before the Deathknell trigger resolves, but this does not affect the outcome since the trigger still waits until after combat ends
• The HOT FEPR system requires all outstanding tasks in the Resolution Step to complete before returning to FEPR