
Hello hello!
Here's a quick primer on justice in Skyhold (aka what to do if a big bad/personal enemy from your world shows up.)
1. If you spot someone you feel may be a threat (and the other player agrees to let you take action!), you may try to apprehend them using reasonable force. Be prepared to justify the amount of force you use, though!
2. Take them to the Skyhold cells.
3. Turn yourself over to be placed in the Skyhold cells. (A separate cell.)
4. A peer group of 2-3 characters (if possible) will be assembled to judge whether the person you apprehended is truly a threat. If they are, they will be dealt with as fairly as possible. If not, they will be released.
5. That same group will judge whether you are a threat. If you acted out of malice or used near-lethal force when it wasn't necessary, for instance, you may find yourself in the cell for a while. If you acted out of reasonable concern, you will be released. Basically, this is to weed out anyone who may attack someone who they just personally hate but who would otherwise likely not be a threat to the main group. (Bartrand, for example. Varric may want to hit him with a crossbow bolt, but since Bartrand isn't an actual threat, he can't without serious consequences.)
This will hopefully keep the peace even in a state of relative anarchy. Please assume that this information was shared/your characters know and keep on being awesome. :)