Hmm. I really like the concept, really, but I think you might be taking the power slightly too far - for one, it's very combat orientated and secondly I see it as a potentially OP power.
I think, maybe, we could work with this though if we stuck a tonne of limitations on it, essentially slimming the power down.
Why not get rid of the dream/nightmare scenario and create one familiar, with a specific appearance and personality? Then use the limitations for things such as how long it can be summoned for, what is can do, perhaps how far away it can travel, etc. It could still be conjured from your characters dreams, originally, and perhaps the summon could slowly change with his personality/state of mind.