The final challenge for Shelda in Bugsnax is to bring her a Black Razzby from under the ruins. You can find the entrance to the ruins near the large pyramid to the left of Shelda. There is a trick to getting in there, however.
On the ground, you will see a circle of earth with some planks and stones in it. This the entrance and you need to break it open. To do so, put some hot sauce on it and the Buffalocust that is flying around will divebomb it, break it open and you can drop down inside.
You should explore this area fully, opening all the doors with your Snak Grappler because this way you won’t have to do them later. There will be a door near where you dropped down with some plants to either side and a small ledge. Go out there and look left to see the Black Razzby.
To lead it out, throw down your Buggy Ball and cover it with chocolate from your Sauce Slinger. The Black Razzby loves chocolate and will follow it. Now, you need to carefully lead it through the room, making sure to avoid the Incherritos. Use the stones on the ground to avoid them. Lead it the whole way through, using the ledges to gives yourself a good vantage point. Don’t drop down into the room, as this will actually make it far more awkward.
When you get to the end, lead the Black Razzby to the jump pad and this will send it up through another hole, and the quest will be complete. You can now go back to Shelda, who will agree to head to Snaxburg.