Late Act 2
The End of Vaal Things
With the opening of the Vaal Ruins, you're ready to proceed with the rest of Act 2, heading back into the time of myth and the story of the lost Vaal civilization.
The second half of Act 2 is quite linear, with only one (entirely optional) side-zone, the Dread Thicket and only one (mandatory) quest. You can follow it easily enough on your own.
As a result, the guide pauses here: go check the 'Late Act 2' section of your Guided Build before moving on. Enjoy your trip through the Vaal Ruins - and try not to unleash any terrible eldritch horrors while you're at it. Return once you've defeated the end boss of Act 2.