Tonight we had an overall great gaming experience playing "A journey in the dark" Saga Adventure.

However, we came across a -problem trying to presume the interaction of "Prince Imrahil" Ally card with the "The Great Bidge" location.

Prince Imrahil reads: " While there is a hero in your discard pile, Prince Imrahil loses the ally card type and gains the hero card type."

The Great Bidge reads: " Response: When The Great Bridge is explored, discard a hero from play to deal X damage to The Balrog. X is that hero's threat cost. Then, The Balrog loses all keywords for the remainder of the game. Any player may trigger this response. "

Of course I had a hero in my discard pile, so I guess that if I discard Prince Imrahil Balrog loses keywords but I do 0 damage to it.

Another question about changing from ally to hero is about attachments that can be attached to a hero. Are they discarded if Imrahil becomes ally again?

I'm not completely sure what the ruling would be for sacrificing Ally Imrahil, but I would suspect that you are right. He doesn't have a printed threat cost, so I assume that you would deal 0 damage. This problem could also come up if you use The Fall of Gil-Galad, but again I think that his threat cost should be assumed to be zero.

Your second question is easier to answer. Attachments only check whether they have a legal target when they are played, so if you play an attachment that only attaches to a hero, and then you use Fortune or Fate to return the hero to play, Imrahil will keep the attachment, however if it only attached to a hero, it probably won't be very useful to him as an ally.

There's actually one attachment that gets better when Imrahil loses hero status -- Dark Knowledge's response would still work, but it would lose the -1 willpower penalty. There's a few other hero attachments that still function on an ally, but it's very much the exception and not the rule.