Lothos Riftwaker is a level 60 quest giver located just outside the entrance to the Grinding Quarry in the contested territory of Blackrock Mountain. To get to him, walk down the second chain heading towards Blackrock Depths. He is standing in a small chamber right at the entrance. Just talk to him after having completed the attunement quest at any time and to be teleported into the Molten Core. This also works while you are dead and corpse running back. Jumping in the lava outside the window works just the same as talking to him.