If I attack with a 12 Might unit and kill two defending units (including Glasc Mixologist), but my opponent uses Imperial Decree to kill all units including mine, can my opponent then play the same 3 Might unit back to that battlefield?

Ruling: Yes, your opponent can play the 3 Might unit back to the battlefield during combat, and they will win the combat as the only player with units remaining at that location.

Sequence:

• Combat damage is dealt and all units die
• Deathknell triggers become pending on the chain
• The FERP process is skipped initially during the combat damage step
• During the Resolution Step's special cleanup, FERP is inserted after cleanup completes
• Deathknell triggers are finalized and executed during this FERP
• The opponent can play their unit to the battlefield at this point (combat is still ongoing, so the defender has not lost control yet)
• Combat result is then determined
• The defender wins combat because they are the only player with units remaining at the battlefield

Nuances:

• Combat never actually ends before the unit can be played back
• The FERP process is skipped during the damage step but gets inserted during the Resolution Step's cleanup
• Imperial Decree's effect lasts the entire turn, so the replayed unit will also die if it takes any damage