All good ideas, Berethor!
I was thinking they could be used by the Aragorn leader if possible; otherwise, it is Gondor or Arnor liek yous ay. They would be a late game ritual or wonder. Ideally, they would be triggered by something bad in the past, but I think this idea is too hard to make work in practice.
My idea was similar to your first one, that they are a powerhouse unit that are only around a few turns; or possibly, that they are (from a programming standpoint) a one use item like the healing salve in FfH. I was thinking their ability would be to be a stack attack (from a programming standpoint, like a crown of command or pillar of fire from Chalid in FfH) but more powerful as a one use item.
I don't think it is hard to incorporate them. I think the harder part is balance, if Aragorn or Gondor can get it, what do other players get to compensate?
Best wishes,
Breunor