When a hidden Teemo, Strategist is revealed as a defender and its ability activates, does it deal damage and potentially kill an attacking unit before that unit deals combat damage?

Ruling: Yes, Teemo's ability resolves and deals damage before combat damage is calculated or applied, which can kill the attacking unit before it deals its combat damage.

Sequence:

• Teemo is revealed from hidden during combat
• Teemo's ability triggers because it is entering combat as a defender
• The ability resolves, dealing damage to the attacking unit
• If the attacking unit is killed by this damage, it dies before combat damage is calculated
• Combat damage is then calculated and applied

Nuances:

• Teemo no longer triggers on play from hidden, but still triggers when defending
• The attacker/defender status is checked when units enter combat, not just at the beginning of showdown
• Entering combat from reveal counts as entering combat and triggers the ability