Well instead of a villain in the terms your currently thinking lets stop and look at the world as a whole. Who would want the void to open, What group and or group of people would this give and advantage too?
Remember your would is built right now on one level and that is the area in which we are currently looking at. Right now its the destoryed castle, which Dragon (Salamander hue hue hue) has currently just crushed. Why did he do this, what did he gain from doing this, what will the characters get from aiding/hindering the current threat.
You need to sit down and ask the big 5. Who, what, when, where and why. From there build a plan that leads into a more open world or lined path. At this point you have things locked in with the current attack. No one really knows why the Dragon just came and stole those people, no one knows why the Golems are attacking.
As far as it stands a rather strong summoner or group of them would need to be around for something like this to have happened. If we go off of that we have a who (sort of you could say its a cult), why are they/this person attacking Mid-haven in the day time. What is the goal or what is the group/person gaining from this attack? (Fellow WoW lore nerds would remember this might ring around the Undead Scourge when they made the black scar to Silvermoon in an attempt to get their hands on the sun well while also making a army of undead to aid them)
We know who the Dragon is, but why did he do what he did. What is he gaining from doing this?
As far as the finial enemy, since we can't get to that yet we leave it as a blank for now. So go ahead and work on the big 5. Most of them we already have, its the other parts that need the work to get this 'player motivation' high. Worse case you could always break down and ask people to get an idea of what they feel is right and or what they want to see. PM/Steam/Discord/Skype it out to people to get feed back in private this way they don't feel pressured to answer out right here if they don't want people seeing what they are thinking and or don't feel comfortable saying it in the general public.