The text is:

"When Revealed: The first player (choose 1): deals 1 damage to all allies in play and Followed by Night gains surge, or all enemies engaged with players make an immediate attack, if able."

I think it is clear that you can choose, otherwise the "choose 1" bit makes no sense. The "if able" bit worries me a bit, to be honest. In German the card is worded like this: "The start player chooses one of the following: …" which implies that the player is always able to decide which of the two options he's going to choose.

Edit: crosspost!
 

No, I think the choice part is clear, it's the "if able" bit that also confuses me.

Dam said:

As starhawk said, there are enemies that are engaged with a player, but not able to attack. Right now only Forest Snare and Thicket of Spears prevent enemies from attacking, but there might come other cards with similar effects.

I don’t think Followed by Night is confusing. The “if, able” applies to enemies. If they are caught in a Forest Snare they can’t attack. If they are not engaged with any player then they can’t attack. If I recall, the enemies do not engage a player in order to make an attack.