Negating a power isn't a problem, and that's a huge "if" when it comes to considering locations for the meeting.
Each of the Underworld leaders are more than capable of killing or injuring someone with their powers because they have the skills to do so.
that being said, I still don;t see the reason for bringing bodyguards, who would also have to enter into this supposed chamber that negates powers and be under the same condition. It would be better for what ever neutral area there is, to provide their own guards to the meeting. However, any information we have at all on the council and the meeting area is extremely nonexistent at this point in time. It could be anywhere from Satan's bedroom to a barren rock covered field. Making any attempt to guess on what is going to happen there, what will be there, what conditions will be like, etc, is stupid at best.
“Salvation comes with a cost. Judge us not by our methods, but by what we seek to accomplish.”
"To punish and enslave..."