Blackrock Deeps Entrance. Starts from Maxwort Uberglint at. It played a large part in the pre-expansion end-game back when the level cap was 60.
The NPC panel has been upgraded to allow some specific filtering you can. I have beaten the Blackrock Challenge twice but cannot exit the Arena where all the battles take place. Another massive case solved just days.
Once inside your map changes and you should be able to see the entrance to Blackrock Depths.
It contains a multitude of bosses and quests that range in difficulty from the low 50s right up to 60 and back then was the first instance where a substantial amount of the loot would last into the post. However there are not two different portals so when you are queued up for either you will start out in the main entrance. The depths have many sections including a prison Shadowforge City a vast capital of the Dark Iron dwarves and the Grim Guzzler tavern. The last part is where I got confused and so did others.