Does Teemo Strategist activate its effect (reveal top 5 cards and recycle them) when played from Hidden in a battlefield with no enemy units to target?

Ruling: No. Teemo's ability cannot enter the chain if there are no valid enemy units to target, so the reveal and recycle effects do not happen.

Sequence:

• When activating any ability in Riftbound, all targets must be declared as the ability is placed on the chain
• If no legal targets are available at that time, the ability cannot be placed on the chain at all
• Since Teemo's ability requires choosing enemy units to deal damage to, and none are available, the entire ability (including the reveal and recycle) cannot activate
• This is true even though the reveal happens first when resolving - targeting requirements are checked before the ability enters the chain, not during resolution

Nuances:

• If the ability successfully enters the chain with valid targets, but those targets become invalid before resolution, you still resolve as much as possible (reveal cards, recycle them, but deal no damage)
• The distinction is between "entering the chain" (requires valid targets) and "resolving" (do as much as you can)
• Teemo will receive official errata/clarification from Riot in the future