When Teemo comes out of hidden and does the top 5 cards hidden damage, and he uses it on a unit and brings that unit's might to 0, does that unit still get to attack in the showdown?

Ruling: Damage doesn't change a unit's might. If you deal damage equal to or greater than a unit's might, it will die. If it dies before the Combat Damage Step, its might won't be calculated in that because it's dead.

Sequence:

• Teemo's ability deals damage when he comes out of hidden
• If the damage equals or exceeds the target unit's might, that unit dies
• Dead units don't participate in the Combat Damage Step

Nuances:

• You cannot cast Rune Prison in response to Teemo's ability because Rune Prison is an action and can't be put on the chain when other things are on the chain
• Stun doesn't affect ability damage, only Combat Damage (the moment in combat where you declare where the might of your units go onto your opponent's units)