» Fri Nov 18, 2011 4:44 pm
The "rules" for passage between the realms seems to be that a resident must invite the outsider. A conjurer can summon Deadra. A resident such as the player can request a portal be opened so they can travel to (e.g.) Peryite's or Boethiah's worlds only at that Prince's invitation. Sheogorath opened the gate on the floating island, but could not send anyone through it. That was an invitation for travel in the other direction.
The Mythic Dawn opened the Gates from the Tamriel side in large numbers, and all over the map. They were assisted by the weakening of the constraints on Mehrunes Dagon resulting from the Dragonfires being unlit, but it still took action from the inside. The MQ climix left it much harder to do, and impossible on the scale of an invasion, but a bit of tourism or inter-planar commerce is still on the table.