The Underworks and The Cauldron
This part of the Underworks is filled with hazards, so you need to be ready. Basically, there are platforms made of scorching lava. If you stay on them for too long, your character will take damage. But if you keep jumping, you should remain safe. There are also several exploding enemy mobs and lava falls from ceilings.
If you make it to The Cauldron, all you need to do is keep heading left as you cross the lava pool. Use your air dashes and pogo maneuver to avoid getting burned to death. Once you’re next to the altar, you can interact with it to receive the Clawline.