Can you target units with Challenge at an opponent's base, and does it start a showdown?

Ruling: You can target units anywhere on the board with Challenge, and it does not start a showdown. The only way to respond is by casting a reaction while Challenge is on the chain.

Nuances:

• Damage dealt by Challenge persists on units unless healed
• A unit's might value does not decrease when it takes damage; it still deals full might damage in showdowns
• A unit dies when its current damage equals or exceeds its might value