Any time and place during the Oblivion Crisis where gates opened up anywhere. This is why I can barely stand to side with Imperials in Skyrim. They had a hold over all continents and races, and when the Oblivion Crisis came to a head, they abandoned every land they stretched their influence to in favor of protecting their homeland. The Dunmer, my favorite race, were not fighters at this point in history. House Redoran fought for years to create a viable militia and ended up creating an extremely powerful army to fight the Daedra. This ended up benefitting the Dunmer for generations to come, obviously. For example, when The Saxhleel (what the Argonians call their race) attacked the Dunmer on their home land they were able to defend themselves and stop the Saxhleel from pushing them to extinction. Saxhleel presumably attacked the Dunmer because they kept the Saxhleel as slaves for a very long time throughout history. Treated them like absolute garbage as well.