This card:

With this text: While Fell Rider is in the staging area, it gains: "Forced: At the beginning of the encounter phase, the first player must either reduce the Ring-bearer'slife by 1 or engage Fell Rider."

Can first player choses? Or he must resolve first part, and if he can't then second part?

I know card with the same structure text that you must resolve first if you can... (and 2nd if not)

For example:

With text: When Revealed: Each hero must pay 1 resource or take 1 damage. If no hero takes damage from this effect, Men in the Dark gains surge.
Shadow: Attacking enemy gets +1 [Attack] for each hero the defending player controls with no resources.

You would have to be able to choose, would you not? If you cannot reduce the Ring Bearer's life, that means he is dead and the game is over. So it's got to be a choice.

It looks like the operative word on Fell Rider is "either." Maybe they are using "either" to confer a choice.

Ok, thanks for the answer.