Warcraft lore question ?

[Chucky]

If the dreadlords where able to enter Azeroth than how come Archimonde had to wait for Kel'Thuzad to open a portal ? Where do they live, the demons ? I know that Medivh opened a portal for orcs to come to Azeroth but couldn't The Burning Legion go through the gate as well ? I'm making a campaign and I need to know this part of lore.

 

[Sky Green]

That's a tough one, but I don't think that Medivh had the power to open a portal to the Twisting Nether not even with Gul'Dan's help, and the Nathrezim shiz I don't know

 

[Edhel-dur]

Here