Hello again,
I went and discussed this with my team again and I spoke specifically with a colleague who keeps regular contact with the developers of D&D. From this discussion, I found that unfortunately, the last answer I gave you is not correct. The first answer I gave you is correct. Now I understand, how frustrating this must be for you now that I have given you two different answers, and again I sincerely apologize for the confusion that I have caused. I am 100% sure now of the answer that I am about to give you. The developers intended that the creature is completely aware of what the power does that is effecting him. However, instead of coming up with rules for each power that states what portions of the power are known and which portions of the power are unknown to a creature, they intended for the DM to use their discretion to fit their game.
So, as the rules are written, and as intended, if you attack a creature with the Dance of Death power, that creature knows exactly how that power works. Now the DM can choose how much knowledge the creature has about that power. If they feel the game would be more fun, or if it would fit their campaign better to have the creature not know all of the effects of a power they can certainly choose to play it that way.
Again I apologize for the confusion that I have caused.