Remember how you jumped into the hole at the top of Icecrown Citadel with the intention of rescuing the people stolen? Yeah, Bolvar is the only one that seems to remember that. Once a week he has a quest to go into Torghast (well, there’s the occasional little bit in the Maw). Some of the Torghast quests can be done in any layer, so you can do them while you get soul ash or just in layer 1 if you don’t care. In some cases he’ll give you a crystal that you use on the pedestal (wayfinder) in Torghast. Those go into a special “layer 0” version.
Bolvar’s quests are limited to one per week. Some of the quests are autobestowed on entering Torghast if you didn’t stop by to see if he had a quest for you, which is nice.
Once you complete Bolvar’s quest chain and have completed layer 8 of one of the regular Torghast wings, the Twisted Corridors version of Torghast opens up. These are 18 floors per layer instead of 6, with the same pattern of 2 floors of enemy and then a broker floor. It also comes in layers, with Twisted Layer 1 being supposedly harder than normal layer 8. Once you complete all 8 layers of that you get a mount usable in the Maw. Or so I’m told.