I thought that once the dragonfires were no longer lit, after a short time any daedric prince could open up stable links between Nirn and Oblivion and begin invading? According to ESO's story, however, despite the fires remaining dark for several years prior to the soulburst, Molag Bal could only invade when Varen attempted the ritual to make himself dragonborn and caused the soulburst. Why couldn't Molag Bal and the other princes simply invade without the soulburst occurring? According to the events of Oblivion, this whole idea of a "soulburst" appears to be lore-breaking.
Sanguine's Beta Tester